Vermont Sen. Bernie Sanders leveled his harshest criticism yet at Hillary Clinton Saturday, as the two spoke at a crucial Democratic dinner in Iowa.

Speaking at the Iowa Democratic Party’s Jefferson-Jackson Dinner, Sanders implied that the former Secretary of State was a political shapeshifter who changes her views for political convenience.

Though he never said Clinton’s name, it was clear that much of his speech was aimed at her.

“I promise you tonight as your president I will govern based on principle not poll numbers,” Sanders told the crowd of about 6,000 Iowa Democrats to raucous cheers. “My message to you today is the same as it was yesterday, and will be tomorrow.”

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Sanders takes pride in the fact that he has never run a negative ad in a political career that spans three decades. His campaign for the Democratic nomination has focused on issues such as addressing income inequality and regulating Wall Street rather than contrasting himself with Clinton. But Tuesday represented a significant turn in Sanders’ rhetoric.

During his 25-minute remarks, Sanders ticked off a long list of areas where he has a more progressive record than Clinton, mentioning his opposition to the Iraq War, the Keystone Pipeline and the Trans-Pacific Partnership, a massive 12-nation trade deal. In cases where their positions are not far apart, Sanders contrasted his long-held views with Clinton’s more recent pronouncements. On the Trans-Pacific Partnership, he quoted a 2012 speech in which she called it the “gold standard” of trade agreements.

“That agreement is not now, nor has it ever been, the ‘gold standard’ of trade agreements,” Sanders said. “I did not support it yesterday. I do not support it today, and I will not support it tomorrow!”

Sanders then brought up Defense of Marriage Act, passed under president Clinton, which defined marriage as being between a man and a woman. 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 constituion and outlaw same sex marriage.

Read More: Clinton Sharpens Her Barbs Against Sanders

“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.”

He also recounted his opposition to the Iraq War in 2002, which Clinton voted for.

“Let me tell you that I listened to what Bush had to say, to what Cheney had to say, to what Rumsfeld had to say,” Sanders said. “I didn’t believe them and I voted no.”

Sanders advisors have hinted in recent weeks that he may be willing to be more critical of Clinton. He has refused to criticize her use of a private email address and will not attack her character, advisors say, but Clinton’s positions on his pet issues are fair game.

The remarks came just a day after Clinton took her sharpest jab at Sanders, implying that he had made a sexist remark at the Democratic debate when he referred to “shouting” about gun control.

Sanders chose a big stage to make his remarks: the Jefferson-Jackson Dinner is a key event in the Democratic primary that paved a path for then-Sen. Barack Obama to win the Iowa caucus.

He also compared his campaign to Obama’s in 2007, saying that Obama too had been considered “unelectable.” Sanders said a principled Democrat could win in 2016.

“I pledge to you that every day I will fight for you, not the corporate interests!” Sanders said at the close of his remarks. “I will not abandon any segment of society, black or white gay or straight, just because it is expedient at a given time.”

Read More: The Gospel of Bernie

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