Former U.S. senator and Rhode Island governor Lincoln Chafee detailed how he would tackle a number of key issues facing the United States including switching to the metric system, if he was elected president in 2016. (AP)

This story has been updated.

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, announced Wednesday that he will seek the 2016 Democratic presidential nomination.

“Today I am formally entering the race for the Democratic nomination for President,” Chafee said during a foreign policy speech at George Mason University in Virginia Wednesday evening. “If we as leaders show good judgment and make good decisions, we can fix much of what is ailing us.”

Chafee's decision to run makes him the fourth Democratic hopeful to officially enter the race. But he's already been the first to directly attack front-runner Hillary Clinton -- particularly over the Iraq War vote that helped sink her first presidential bid.

“It’s heartbreaking that more of my colleagues failed to do their homework. And incredibly, the neocon proponents of the war who sold us on the false premise of weapons of mass destruction are still key advisers to a number of presidential candidates today...including the main Democratic candidate," he said.

Although Chafee did not mention Clinton by name during his speech or in the question-and-answer session that followed, he pointed to several recent controversies surrounding the Democratic front-runner. "Our State Department just has to be above all controversy, and it’s regrettable now to me what’s happening with e-mails [and] the foundation,” he said when asked about the former secretary of state.

Chafee, the son of Rhode Island Sen. John Chafee (R), was appointed to the U.S. Senate after his father died in 1999, leaving the seat vacant. As a liberal Republican, Chafee was reelected 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 is hoping it will once again take center stage in the Democratic contest, too.

"Don't forget that probably the biggest reason that Senator Obama, at the time, defeated Hillary Clinton in '08 was because of the Iraq war vote. That was the issue," Chafee told CNN in April. "And that's my big issue here, because we are dealing with [the] ramifications of that huge mistake that Senator Clinton made in 2002, which I did not make, and we live with it today."

[Lincoln Chafee accuses Hillary Clinton of diplomatic clumsiness, says 'America loves an underdog']

Clinton's other rivals for the Democratic nomination have so far mostly stopped short of direct attacks on the former secretary of state. Former Maryland governor Martin O'Malley (D), who last week announced his candidacy, has made subtle digs that focus on Clinton's perceived coziness with Wall Street and her long history in Washington. Sen. Bernie Sanders (I-Vt.) has staked out populist positions that sometimes fall far to Clinton's left, but he takes pains to avoid mentioning her explicitly.

Chafee does not appear concerned with subtlety.

"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."

Several weeks ago, he suggested that heightened tensions with Russia might be traceable to a symbolic, incorrectly labeled "reset button" that Clinton presented to that country's leadership several years ago.

Former U.S. senator and Rhode Island governor Lincoln Chafee is a Democratic contender for the White House in 2016. Here's his take on Iraq, Obamacare and why he changed parties, in his own words. (Gillian Brockell/The Washington Post)

"In the early days, they tried to restart with Russia and she presented the Russian foreign minister with the restart button. And they got the Russian word wrong. They said, 'This means over-charge,' and it was an insult," he said. "Look what is happening with Putin and with Russia -- Ukraine, selling arms to Iran -- and it all could have started with the diplomatic mistake, getting the word wrong."

Chafee has said that he does not expect to raise nearly as much money as Clinton but that he takes comfort in knowing that "America loves an underdog."

In a year when one of the candidates seeking the Democratic nomination isn't even a member of the party, Chafee isn't concerned that his past party allegiance will raise eyebrows in his relatively new partisan home. "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," he said in April. "Also note that of the candidates here, [former Virginia senator] Jim Webb was a Republican and Hillary Clinton was a Goldwater Girl."

A Washington Post/ABC News poll released Tuesday suggested Clinton has the support of 62 percent of likely Democratic voters nationwide, with Sanders at 10 percent and O'Malley at 3 percent. Chafee registered 1 percent support.

This story has been updated to correct the 2006 election result: Chafee did not win reelection in 2006.