Responding to a 2007 Boston Globe survey of presidential candidates, Barack Obama rejected the idea that the president has the authority to wage war without congressional authorization whenever he thinks it is in the national interest. "The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation," he said.
After he was elected president, Obama took a different view, maintaining that he did not need congressional approval to attack Libya, Syria, or ISIS. He also defied the requirements of the War Powers Resolution, absurdly arguing that dropping bombs and firing missiles from drones in Libya did not constitute "hostilities" under that law.
Given that experience, we probably should take statements on this subject from the 2020 presidential contenders with a grain of salt. It is nevertheless instructive to see how they responded to a New York Times survey about executive power, since they express a pretty wide range of views and what they say now can be held against them later. The 14 Democrats and two Republicans who responded to the survey can be roughly divided into three groups: four who are signaling that they will do pretty much whatever they want when it comes to deploying the country's military might, eight who argue that the president's war powers are more constrained than residents of the White House historically have been willing to admit, and four who fall somewhere in the middle.
Article I of the Constitution says Congress has the power "to declare war," while Article II makes the president "commander in chief" of the armed forces. Recent presidents have taken the position that the latter power effectively nullifies the former power, making it unnecessary to seek a declaration of war even in situations where the country does not face an imminent or actual attack.
The Times specifically asked the presidential candidates about the view, backed by the Justice Department's Office of Legal Counsel (OLC), that "the Constitution authorizes the president, as commander in chief, to order the military to attack other countries without congressional permission if the president determines that this would be anticipatory self-defense or otherwise serve the interests of the United States—at least where the nature, scope and duration of the anticipated hostilities are 'limited.'" The paper also asked, "Under what circumstances other than a literally imminent threat to the United States, if any, does the Constitution permit a president to order an attack on another country without prior Congressional authorization?" And it wanted to know whether the candidates thought "bombing Iranian or North Korean nuclear facilities" would require congressional approval.
Here are the most telling responses, divided into unconstrained and constrained views of the president's war powers.
Former Vice President Joe Biden (D): "The Constitution vests the President, as Commander in Chief and Chief Executive, with the power to direct limited U.S. military operations abroad without prior Congressional approval when those operations serve important U.S. interests and are of a limited nature, scope, and duration…Only in the most exigent circumstances would I use force without extensive consultation with Congress." Since "U.S. interests" are in the eye of the beholder and the president unilaterally decides when military operations are "limited" enough that they do not qualify as "war," this formulation amounts to a blank check. And notice that Biden promises "consultation" with Congress, which is emphatically not the same thing as seeking formal approval.
Sen. Kamala Harris (D–Calif.): "The President's top priority is to keep America secure, and I won't hesitate to do what it takes to protect our country in the face of an imminent threat in the future. But after almost two decades of war, it is long past time for Congress to rewrite the  Authorization for Use of Military Force that governs our current military conflicts. The situations in Iran or North Korea would require careful consideration of all of the surrounding facts and circumstances."
Sen. Amy Klobuchar (D–Minn.): "I agree with the Office of Legal Counsel's determination that the President is authorized to direct the use of force where the nature, scope and duration of the anticipated hostilities are limited. I also believe that the President has a solemn duty to protect and defend the United States and that the Constitution requires Congress to authorize war."
Rep. Tim Ryan (D–Ohio): "The President has a unique obligation to defend the country and it should be handled on a case-by-case basis on the actual security risk it poses to the country."
South Bend, Indiana, Mayor Pete Buttigieg (D): "I am concerned that the Executive Branch has stretched the President's unilateral war-making authority too far.….[The OLC's position] acknowledges the reality that a President may need, in rare and extraordinary circumstances, to take swift action in response to attacks or imminent threats of attack. But while it may reflect history, it strays from our Constitution's design. Moreover, it lacks criteria for determining which 'national interests' qualify, as well as any identifiable limiting principles on what constitutes 'war.'"
Rep. Tulsi Gabbard (D–Hawaii): "I do not agree with OLC reasoning. The president is the commander in chief of our armed forces. This gives the president the power to respond militarily to attacks on the United States. Congress maintains the responsibility to declare war. As FDR demonstrated when he asked Congress for a declaration of war after Pearl Harbor, the president must seek congressional authorization for actions beyond an immediate response to an injury of [or] threat of it. Article I, Section 8, Paragraph 11 unequivocally assigns the authority to authorize war to the Congress, not the Executive. We need to maintain military readiness but also respect the separation of powers that is the bulwark of our liberties as Americans."
Former Rep. Beto O'Rourke (D–Texas): "In situations where the use of force is necessary, absent an imminent threat to our national security, I will take that case to Congress and the American people to seek authorization."
Sen. Bernie Sanders (I–Vt.): "While the president has the authority over the conduct of war once it has been declared, the Founding Fathers gave the power to authorize military conflicts to Congress, the branch most accountable to the people. Certainly, [while] the president must have the ability to defend our country from imminent attacks or other extraordinary circumstances, the current guidelines violate the text and spirit of the Constitution, and have been used to justify military action that clearly falls under the authority of Congress's War Powers."
Former Rep. Joe Walsh (R–Ill.): "The OLC opinions on this matter leave too large a door for the executive branch to walk through when justifying unpopular use of force…Congress must have a meaningful role in contexts in which we are considering sending our troops to war. As President, I would commit to seeking congressional authorization before starting armed conflicts in new theatres against new adversaries.…It is inexcusable to exploit a statute passed just three days after the terrorist attacks of September 11th [the 2001 AUMF] to [support] military action abroad without congressional approval."
Sen. Elizabeth Warren (D–Mass.): "There are some situations—such as repelling an attack on the United States and protecting the lives and property of Americans abroad—that have been recognized for more than a hundred and fifty years to allow for the use of force without prior congressional authorization. But in recent decades the Justice Department has significantly expanded that category. These newer justifications, which consider national interests and the anticipated nature, scope, and duration of hostilities, are so broad and flexible that they offer few logical or practical limits and can be used to initiate conflicts that Congress should have to authorize."
Former Massachusetts Gov. Bill Weld (R): "It is imperative that the government of the United States be able to respond swiftly and decisively to hostile threats to our safety, security, and other essential interests. This does not require evisceration of our Constitution's unambiguous assignment of war making powers to Congress. This power has been inappropriately compromised and eroded over a number of successive Presidential administrations and various Congresses in the hands of each of the major parties. The O.L.C.'s reasoning is simply a recent manifestation. The Office of Legal Counsel's contra-Constitutional reasoning and its conclusions are incorrect."
Marianne Williamson (D): "Any offensive use of the military by the President requires prior express congressional authorization.…The President, however, may respond unilaterally in self-defense to repel foreign aggression that has already broken the peace….All of history shows the executive branch chronically concocts excuses for war to aggrandize power and earn a place in history, including limitless power to kill, spy, detain, or torture citizens and non-citizens alike free from congressional or judicial accountability. A presidential war not declared by Congress is the classic definition of an impeachable offense that should result in the President's trial in the Senate and removal from office."
The other four candidates who participated in the survey—Sen. Michael Bennet (D–Colo.), Sen. Cory Booker (D–N.J.), Montana Gov. Steve Bullock (D), and former Rep. Bart Sestak (D–Pa.)—suggested that they take a relatively narrow view of the president's war powers. But Bennet said his view is only "slightly more narrow" than the OLC's, Bullock did not say when congressional approval is required, and Booker and Sestak both left the door open to unapproved military action in defense of "interests."
Judging from this survey, the most full-throated critics of unconstrained presidential war making are Gabbard, Sanders, and Williamson, although Warren and the two Republicans are not far behind. And since presidents are unlikely to be less interventionist than they indicate during their campaigns, people who are appalled by what the military does in the name of "defense" should be wary of candidates who talk like Biden, Harris, Klobuchar, and Ryan.
Do these words matter? Maybe not. "The next Democratic president will happily accept new rules on tax releases, but will have a harder time accepting constraints on security clearances and emergency or war powers," Harvard law professor Jack Goldsmith, who ran the OLC during the George W. Bush administration, told the Times. "Institutional prerogative often defeats prior reformist pledges."
A bigger problem than presidents who break campaign promises to restrain themselves is a Congress that refuses to restrain them. The War Powers Resolution was enacted in 1973, but Congress did not even try using it to curtail a president's military adventures until earlier this year, when both houses approved a resolution condemning U.S. participation in Yemen's civil war. Although it was ultimately vetoed, that resolution, combined with the resolution against Donald Trump's declaration of a border wall emergency (also vetoed), provided some reason to hope that the current president is erratic enough to awaken a slumbering Congress.
"I will encourage members of Congress to reassert and exercise their constitutional role in decisions to commit their constituents and communities to supporting our military operations," Buttigieg said in the Times survey. The fact that Buttigieg, who takes a relatively constrained view of presidential war powers, did not leave open the possibility that Congress might decline to support military operations favored by the president speaks volumes about the legislative branch's abdication of responsibility.