Former Rhode Island governor Lincoln Chafee is set to officially launch his bid for the Democratic nomination during a speech delivered this evening at George Mason University. Chafee, former Republican, turned independent, turned Democrat, has been a strong critic of Hillary Clinton since he began actively exploring the possibility of a presidential run a few months back.

Report from the Washington Post:

Former Rhode Island governor Lincoln Chafee (D), a one-time Republican U.S. senator who notably broke with the GOP on the 2002 Iraq war authorization, is expected to announce Wednesday that he will seek the 2016 Democratic presidential nomination.

Chafee’s decision to run, which he plans to announce in an evening speech at George Mason University, makes him the fourth Democratic hopeful to officially enter the race. But he’s already been the first to directly and consistently attack frontrunner Hillary Clinton — particularly over the Iraq War vote that helped sink her first presidential bid.

“I don’t think anybody should be president of the United States that made that mistake” of voting for the Iraq War, Chafee told The Post in April. “It’s a huge mistake, and we live with broad, broad ramifications today — of instability not only in the Middle East but far beyond and the loss of American credibility. There were no weapons of mass destruction.”

Chafee, the son of Rhode Island Sen. John Chafee (R), was appointed to U.S. Senate after his father died in 1999, leaving the seat vacant. As a liberal Republican, Chafee was re-elected to a full term in 2000, before losing in 2006. He officially left the Republican Party the following year. He was elected governor as an independent in 2010 and joined the Democratic Party in 2013. His announcement Wednesday comes less than two years after he decided not to seek a second term as governor amid low approval ratings and the prospect of a bruising primary.

Central to Chafee’s presidential campaign: his 2002 Senate vote against authorizing the use of military force in Iraq in 2002, when he was the only Republican senator to oppose the measure.

Instability in Iraq has become an unexpected campaign issue in the GOP presidential race. Chafee’s hoping it will once again take center stage in the Democratic contest too.

As noted, Chafee was one of a handful who voted against the use of force in Iraq puts him in contrast to then-Senator Hillary Clinton, who voted in favor of it. Chafee is hoping to capitalize on this during the primary with the intention of drawing in the progressive, anti-war wing of the party to unite against Hillary.