Sen. Bernie Sanders (I-Vt.) turned up the heat on former secretary of state Hillary Clinton over the weekend.

During a 25-minute address Saturday night at the Iowa Democratic Party's all-important Jefferson-Jackson dinner in Des Moines, the Vermont senator promised he would not "equivocate" as president and insinuated that Clinton would.

Bloomberg:

"I will govern based on principle, not poll numbers," Sanders said ... before detailing his staunch opposition to the Defense of Marriage Act, the Iraq war, and major trade agreements — all examples that highlight Clinton's shifting positions over the years. His attack echoed then-Senator Barack Obama’s assertion at the same dinner in 2007 that Democrats have "made the biggest difference in the lives of the American people when we led, not by polls, but by principle.”

Sanders did not mention Clinton by name, but his target was clear. Speaking about the Trans-Pacific Partnership, a trade deal Clinton recently joined him in opposing, Sanders said he had never viewed it as the "gold standard of trade agreements" — words she used to describe it in a memoir. 

The Vermonter highlighted his vote against the Iraq War, which Clinton supported, and his opposition to two measures Clinton's husband, former president Bill Clinton, signed into law: the 1996 Defense of Marriage Act and the 1999 repeal of Glass-Steagall Act banking regulations. 

Time:

Hillary defended her support of it in an interview with MSNBC on Friday, calling it a defensive line against Republicans who wanted to amend the constitution and outlaw same sex marriage.

“Today, some are trying to rewrite history by saying they voted for one anti-gay law to stop something worse,” Sanders said, clearly referring to Clinton. “Let us be clear. That’s just not true. There was a small minority opposed to discriminating against our gay brothers and sisters. Not everybody held that position in 1996.”

Sanders wasn't alone in striking a tough tone, as the Des Moines Register noted:

And in a dig at her chief rival, Bernie Sanders, [Clinton] said: “It’s not enough to just rail against Republicans and billionaires. We have to win this election.”

And at the Jefferson-Jackson dinner, Clinton again deployed an attack she debuted Friday, insinuating that Sanders was engaging in sexism when he said at a recent Democratic debate that "all the shouting in the world" would not prevent gun violence. 

“I haven’t been shouting, but sometimes when a woman speaks out, some people think it’s shouting,” the New York Times quoted her as saying at the Jefferson-Jackson Dinner. 

Sanders brushed off that charge with a laugh Sunday during an appearance on CNN's "State of the Union."

"Well, you know, all that I can say is I am very proud of my record on women's issues," he told moderator Jake Tapper. "I certainly do not have a problem with women speaking out. And I think what the secretary is doing there is taking words and misapplying them."

The campaign's newly-negative tone emerged after several important developments last week: Vice President Joe Biden's declaration that he would not join the race — and former senator Jim Webb's and former governor Lincoln Chafee's decisions to drop out. The Democratic primary is now effectively a two-person race between Clinton and Sanders, with former Maryland governor Martin O'Malley still failing to gain traction.

Sanders's new approach is not without risk, according to Politico:

The shift represents a gamble: Can a nice-guy candidate publicly dedicated to running on substance turn to attack mode without sacrificing his reputation as an authentic voice of the people?

Washington Post chief correspondent Dan Balz asked much the same question:

Sanders drew enthusiastic support from his supporters, who filled one side of the arena Saturday, but in launching this criticism of Clinton, he faces a pair of challenges. One is obvious: If he and Clinton now agree on many of the issues the senator from Vermont highlighted Saturday, will Democratic voters reward him just because he got there first? The other is how Sanders takes what he said Saturday and turns it into a positive and ­forward-looking message that can expand his base of support.