Two key endorsements from former Democratic rivals and the announcement of impressive fourth quarter fundraising totals helped to buoy Harvard Law School professor Elizabeth Warren’s campaign for the U.S. Senate this week.

In separate announcements, former Democratic hopefuls Robert K. Massie and Alan A. Khazei ’83 both said they would endorse Warren in her run against Massachusetts Sen. Scott Brown this fall. Both candidates withdrew from the Democratic nominating race shortly after Warren announced her candidacy last October.

The endorsement of Warren by her two chief Democratic rivals signals growing unity from Massachusetts Democrats, which analysts said will be key if their party wishes to unseat Brown.

“The endorsements are more about the Democratic Party getting behind Warren than delivering votes,” said Mass. Democratic strategist Mary Anne Marsh in an email, adding that Democrats now recognize Warren as the only candidate among their ranks able to take on Brown.

The endorsements came as Warren’s campaign announced new fourth quarter fundraising numbers, which exceeded those of Brown for a second consecutive quarter.

Warren raised $5.7 million in the fourth quarter of 2011 alone compared with Brown’s $3.2 million for the same time period, according to disclosures from both campaigns. Yet despite raising $8.8 million since she declared her candidacy last September, Warren still trails the well-backed Republican by almost half.

Brown raised a relatively small $8.5 million for the whole of 2011, but the large cash reserve left over from his 2010 Senate run continues to give him an advantage over Warren. At the end of the fiscal year, Brown had $12.8 million cash on hand and Warren roughly $6 million.

The financial disclosures and early endorsements show that Warren’s campaign has gained momentum both in Massachusetts and nationally, according to former Boston City Councilor Lawrence S. DiCara ’71.

“There’s been great interest with people who traditionally do not participate,” DiCara said. “I think her candidacy is exciting a lot of people.”

Enthusiasm for the Harvard professor and consumer advocate has rallied support within the state, but national Democratic leaders and forces have been drawn to the race by its broader implications.

“This may be one of the very few opportunities for Democrats to flip a seat this election,” DiCara said. He explained that the same anti-establishment movement that swept Brown into office two years ago in a special election could potentially hurt him as an incumbent this time around.

“The law of politics is the law of the pendulum, and perhaps the pendulum is swinging back,” DiCara said.

Warren’s reputation as a consumer advocate and her work with the Consumer Protection Bureau may allow Warren to take back some of the decisive middle class vote Brown received in last election, DiCara and Marsh said.

“The 2012 election will be based on one thing: whom do you trust to fight for you in Washington? Scott Brown … or Elizabeth Warren who has a record of fighting for middle class families in Washington,” Marsh wrote.

The $5.7 million raised by Warren in the fourth quarter came from 23,000 residents of Massachusetts and a host of other national donors, according to her website, which identifies her fundraising efforts as a “grass-roots” campaign to take down Brown’s “war chest.”

“For a first time candidate to raise more than an incumbent United States Senator is impressive,” Marsh wrote. “What is more impressive is the number of donors and the fact that the average contribution is $64.”

“Regretfully, money is a barometer and the fact that she’s raised so much money is indicative that she is being taken seriously,” DiCara said, comparing Democratic fundraising effort today to the one made to launch Sen. John Kerry to the White House eight years ago.

—Staff writer Nicholas P. Fandos can be reached at nicholasfandos@college.harvard.edu.