Bernie Sanders went after Hillary Clinton's record on Iraq at a CNN town hall event Monday night, attacking Clinton's vote to invade more aggressively than in previous debates.

"I have tried — as I hope you all know — not to run a negative campaign ... to keep this discussion on a high level, where we debate the issues facing this country," Sanders said, standing up from his chair at the forum, held in Iowa a week before primary voting begins.

"The truth is that the most significant vote and issue regarding foreign policy that we have seen in this country in modern history is the vote on the war in Iraq," Sanders said. "I voted against the war in Iraq ... Hillary Clinton voted for the war in Iraq."

Sanders was asked if he was prepared for the presidency, and responded by pointing to the judgment he showed in voting against the Iraq War.

"I said, 'No, I think that war is a dumb idea.'"

An increasingly harsh line of attack on foreign policy

Sanders has tied Clinton to her Iraq War vote previously during the campaign, but his comments about her during the forum on Monday night were the kind of pointed attack some political observers have said he should go for.

Earlier this month, the New York Times's Ross Douthat argued that if Sanders really wanted to win the race, he would need to highlight — and more sharply criticize — Clinton for her Iraq War vote.

Vox's Matt Yglesias suggested before the first Democratic debate that Sanders make foreign policy an issue:

This is not a traditional area of emphasis for Sanders, but it's one that he would be wise to try to exploit, since it is an area of emphasis for Clinton. It's also an issue where she's out of step with her party's base.

But Sanders hasn't always made the contrast between his own record and Clinton's explicit before Monday night.

Here's the Vermont senator, for instance, during the third Democratic debate, according to the Washington Post's transcript:

Look, the secretary is right. This is a terribly complicated issue. There are no simple solutions. But where we have a disagreement is that I think if you look at the history of regime changes, you go back to Mossaddegh (ph) in Iran, you go back to Salvador Allende who we overthrew in Chile, you go back to overthrowing Saddam Hussein in Iraq, you go back to where we are today in Syria with a dictator named Assad. The truth is it is relatively easy for a powerful nation like America to overthrow a dictator but it is very hard to predict the unintended consequences and the turmoil and the instability that follows after you overthrow that dictator.

In this debate, held in December 2015, Sanders didn't agree with Clinton on everything. But the criticism is clearly blunted — he noted that foreign policy is "terribly complicated" and that there are "no simple solutions."

In his Times column, Douthat imagined what a harsher line of attack would sound like from Sanders:

"Look, as a senator Hillary Clinton cast a reckless vote for the Iraq war, which cost thousands of American lives and led directly to the rise of ISIS. As secretary of state she treated national security cavalierly, exposing classified information to our enemies, and also led us into a second disaster in Libya ..."

On Monday, Sanders didn't go that far. But in explicitly and frontally highlighting her Iraq War vote, Sanders took on what many regard as a big vulnerability for Clinton — perhaps reflecting a growing confidence that he has a chance to win the race.