The argument is simple. The history, more complicated.

Bernie Sanders argued again during the first two-candidate debate of the primary that Joe Biden is the wrong man for the White House because, among other sins, he supported the Iraq War. Sanders voted against authorizing that conflict while the then-senator from Delaware voted for it.

The congressional record clearly supports this fact. But Sanders wasn’t content to rest there. Biden’s transgression is compounded, in Sanders’ telling, because he followed a president, George W. Bush, who  knew all along that no weapons of mass destruction would be found there.

“Most people who follow that issue closely knew the Bush administration was lying through its teeth with regard to Saddam having weapons of mass destruction,” the progressive senator said Sunday night. “I understood that -- I was on the floor of the House time and time again.”

It is a popular argument with some audiences, and Sanders makes it regularly on the stump and the debate stage. To drive the point home, he even cut an ad about it ahead of Super Tuesday. The television spot shows Biden and Bush, along with then-Defense Secretary Donald Rumsfeld and then-Vice President Dick Cheney, one after the other, all saying the same four words: “weapons of mass destruction.” The screen then cuts to Sanders, again asserting that he “never believed what Cheney and Bush said about Iraq.”

Interviews and statements that Sanders made at the time, however, do not support this claim. It seems, instead, that the Vermont lawmaker also believed, or took at face value, intelligence reports about the presence of WMDs. He said so. On television.

It was September of 2002, and the U.S. military was focusing on mopping up operations in Afghanistan while the Bush administration trained its sights on Iraq. Patrick J. Buchanan wanted to know if Sanders supported an invasion.

“Saddam Hussein has weapons of mass destruction,” said the conservative MSNBC host, laying out the administration case. He used chemical weapons first on the Kurds and then Iranians and now “is on his way to nuclear weapons.” If the Iraqi dictator had that kind of arsenal, Buchanan continued, “he could use one on the United States, and we just cannot take that risk. We have got to take him out.”

Sanders didn’t protest. He concurred, with one qualification: “I agree with everything you said except the last sentence.”

The real questions wasn’t if the Iraqis had WMDs, Sanders insisted. The issue was “whether we will do more harm than good by acting unilaterally. As you know, virtually every country in the world disagrees with us about the United States acting alone.”

Sanders wanted a coalition of a nations to respond to the threat, and if such a coalition formed, he suggested he might change his mind regarding invasion. “Now if you gave me that sentence and said, ‘Let us act as we did with the Persian Gulf War, with the international community, with the United Nations,’” he concluded, “that becomes a different story.”

This was something of a standard response. Questioned the prior month by the Brattleboro Reformer, Sanders described Saddam Hussein as “a despicable tyrant,” and he “acknowledged reports that Iraq is believed to be trying to develop nuclear weapons,” the paper reported. He still opposed a unilateral invasion, but not because Bush was “lying” about WMD or anything else, for that matter.

“The language of the U.S. Constitution is very clear on this issue and grants Congress, not the White House, the sole power to declare war,” Sanders argued.

Even as the Bush White House continued its drumbeat about the dangers of a nuclear Iraq, Sanders did not change his explanation. One month before American troops crossed the Kuwait border into Iraq, he told Tucker Carlson that the existence of weapons was itself a reason to avoid war.

“Well, the CIA has already told us that if he, in fact, is invaded, as a last recourse he might well use those weapons,” Sanders told the conservative host then of CNN on Feb. 7, 2003. Again, the Vermont congressman argued for an international coalition to deal with Iraq: “My strong feeling is that if the most powerful nation on Earth wages a unilateral invasion against a weak, Muslim nation, it's going to result in an anti-Americanism, instability and more terrorism, not less terrorism.”

The Sanders campaign did not return a request for comment, but a search of the congressional record by both NBC and later RCP shows that Sanders did not criticize U.S. intelligence assessment about WMDs in Iraq until after the fall of the Ba’athist regime. In fact, as NBC first reported, he backed legislation that would have allowed the invasion of Iraq.

Sanders voted for an amendment authored by California Rep. Barbara Lee that would discourage the United States from launching a preemptive strike while bolstering diplomatic efforts to find a peaceful solution. It would not, however, prevent war. Lee stressed that point on the House floor in October of 2002.

“What this resolution does state very clearly and firmly is that the United States will work to disarm Iraq through United Nations inspections and other diplomatic tools,” she said when introducing the legislation. “It states that we reject the doctrine of preemption, and it reaffirms our commitment to our own security and national interests through multilateral diplomacy, not unilateral attack.”

“It does not,” she emphasized, “foreclose any future options.”

The measure would have given U.S. troops the go-ahead to eliminate weapons of mass destruction and long-range missiles in Iraq “by force if required.” The legislation only requested that Bush seek congressional authorization for war if the United Nations could not organize an international coalition.

Sanders voted for the amendment. It failed, but as the New York Times reported at the time, the legislation was largely a signaling exercise. It was meant to provide cover for legislators who worried about appearing too dovish, the paper reported, so that they could oppose an invasion while also arguing “that they supported military action under the right circumstances.”

None of this context makes it into the speeches that Sanders gives today. “Joe and I listened to what Dick Cheney and George Bush and Rumsfeld had to say,” the senator said at the Iowa debate earlier this year. “I thought they were lying. I didn’t believe them for a moment. I took to the floor. I did everything I could to prevent that war.”

But if Sanders didn’t believe the Bush administration’s argument about WMDs, he didn’t say anything until after the invasion. “All of us understood that Saddam Hussein was a thug and a dictator,” Sanders said in June of 2003. “But that's not the reason the president told us we had to go to war.

“Repeatedly, he told us we were in danger of an attack from Iraq's weapons of mass destruction. Now it appears that much of the intelligence reflected significant doubts about the correctness of the assertion,” Sanders said then.

Over time, this morphed into a dubious -- and much more sinister – notion: namely, that Bush and his inner circle “lied” about their reasons for invading Iraq. He’s not the first presidential contender to proffer this particular conspiracy theory. Four years ago, another truculent candidate made similar accusations on the debate stage.

The U.S. military involvement in Iraq was “a big fat mistake,” this candidate said. “They lied,” he added. “They said there were weapons of mass destruction -- there were none. And they knew there were none.”

This candidate’s name, of course, was Donald J. Trump.