Alan Khazei has become the first high-profile Democrat to enter the race to challenge GOP Sen. Scott Brown in Massachusetts.

“Today I’m announcing that I am an official candidate for U.S. Senate in Massachusetts in 2012,” he wrote in a message posted on his website, which details a campaign kickoff event at 6 p.m. Tuesday in Boston and includes an announcement video.

Khazei, a Harvard Law School graduate, is the co-founder of the nonprofit volunteer organization City Year. He ran in the 2009 Democratic primary in the special election to replace one of his mentors, the late Sen. Edward Kennedy (D). And while he earned the endorsement of the Boston Globe, Khazei finished third.

“Everything I’ve done in my life, from starting City Year to saving AmeriCorps and helping pass bipartisan national service legislation that gave purpose focus and a paycheck to hundreds of thousands of people — every time people said, ‘Oh that sounds great, but it can’t be done.’ Yet we did it. Because I’ve always led with an understanding that it’s not about me, it’s about you, and it’s about us,” he said in his announcement video.

Khazei likely will not be the only high-profile Democrat to enter the race against Brown, who is among the Democratic Senatorial Campaign Committee’s top targets in the nation. But despite an electorate that skews heavily Democratic, Republicans consistently cite Brown’s poll numbers that suggest he may be the most popular politician in the state. And he’s sitting on a war chest of more than $8 million.

Republicans were quick to quietly highlight Khazei’s liabilities Tuesday morning.

They noted that his fundraising committee showed more than $190,000 in debt from his 2009 run as of March 31, $115,000 of which was money he personally loaned his campaign. Khazei may also struggle to raise money given his plan to avoid donations from political action committees and lobbyists.

Watch Khazei’s annoucement video: