Hillary Clinton might have a new headache.

On the eve of her presidential campaign launch, Lincoln Chafee, a former U.S. senator and governor from Rhode Island, announced Thursday that he is exploring a run for the Democratic presidential nomination. And unlike other potential Clinton challengers, Chafee appears to be spoiling for a fight.

In an interview with The Washington Post on Thursday, Chafee did not mince words when he said Clinton's 2002 Senate vote to authorize military action in Iraq should disqualify her from becoming commander in chief.

"I don't think anybody should be president of the United States that made that mistake," Chafee said. "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, who was a Republican at the time, was the only senator from his party to vote against the Iraq war authorization. "I did not make that mistake," he said.

Chafee said that foreign policy — including his indictment against Clinton's hawkish record in the Senate and as secretary of state — would be a centerpiece of his presidential campaign.

"I refer to it as brains over biceps," he said. "We've been having the muscular approach that isn't smart in the long term, and certainly the decision to invade Iraq is the biggest mistake of many."

Former senator and governor from Rhode Island, Lincoln Chafee, announced in a video message that he is exploring a run for the Democratic presidential nomination. (Vimeo / Lincoln Chafee)

Chafee also signaled that he would make income inequality a major theme of his campaign and noted that even as a Republican, he voted against then-President George W. Bush's tax cuts. Asked whether Clinton's coziness with Wall Street concerned him, Chaffee said it did.

"We're going to have this debate through the course of the coming weeks and months," Chafee said. "Yeah, we all know how raising money sometimes affects your judgment.”

Chafee described his relationship with Clinton as "professional," noting that they served together on the Senate Environment and Public Works Committee and partnered on some legislation.

Chafee served in the Senate as a Republican, but left the party in 2007 to become an independent. He was an active supporter of President Obama in his 2008 and 2012 campaigns and was elected governor in 2010 as an independent. In 2013, Chafee became a Democrat, though did not seek reelection in 2014 amid low approval ratings.

Asked why Democrats should trust a former Republican to be its presidential nominee in 2016, Chaffee defended his politics — and took another shot at Clinton.

“When you look at my record on all the issues, whether it’s on the environment or on fiscal responsibility or on immigration reform, I’ve been very, very consistent," he said. "I have not changed. My old liberal, Republican stand on the issues does line up with the Democratic Party — women’s reproductive freedoms, support for working families. I have a 30-year record. Also note that of the candidates here, [former Virginia senator] Jim Webb was a Republican and Hillary Clinton was a Goldwater Girl."