(CNN) -

With four months to go until the midterm elections, the Democratic senator in a crucial contest that could decide the balance of power in the Senate, has just over $5 million in her campaign war chest.

Democratic Sen. Jeanne Shaheen of New Hampshire's re-election campaign announced Monday that it had $5.1 million cash on hand. And they reported that the senator raised more than $2.8 million the past three months, bringing to more than $10 million she's brought in this cycle. The campaign said Shaheen received donations from more than 31,000 contributors, up nearly 5,000 from the first three months of the year.

"Senator Shaheen is proud to have such strong grassroots support for her campaign and her work to put New Hampshire first and make a difference for the people who live and work here," said Harrell Kirstein, Shaheen campaign spokesman, in a statement.

Last Monday, the final day of the second quarter, Shaheen got an assist from Democratic Sen. Elizabeth Warren of neighboring Massachusetts, as the progressive rock star hosted a fundraiser for Shaheen in Boston.

Republicans, who need to win back six Democratic held seats this November to recapture the Senate, view Shaheen - a former New Hampshire governor - as vulnerable as she runs for a second term in office.

Last week the campaign of former Sen. Scott Brown of Massachusetts, who is considered the frontrunner among the Republican candidates hoping to challenge Shaheen this November, announced that it raised more than two million from April through June, his first full quarter as a Senate candidate. Brown's campaign didn't say how much money it has in the bank.

The Brown campaign is confident they can keep pace with the incumbent.

"It's clear that both campaigns are going to have the resources they need to get their messages out. That won't be a problem for anyone in this election. Senator Shaheen's problem is her message. Voting 99 percent of the time with President Obama is not a winning formula in New Hampshire in 2014," said Elizabeth Guyton, Communications Director, in a statement Monday.

The Brown campaign revealed their campaign cash figures one day before the candidate was formally endorsed by 2012 GOP presidential nominee Mitt Romney at a high profile event at the site near the New Hampshire seacoast where just over three years ago the former Massachusetts governor officially launched his second bid for the White House.

Monday the Brown campaign released a video of that event, which includes clips of both Romney and Brown, as well as sound from supporters at the event.

"We feel he is our best Republican to win this race and bring a Republican majority to the U.S. Senate, one of the supporters in the video said of Brown.

A poll released last month indicated Brown was the clear frontrunner in the September 9 GOP primary. That survey, and another recent poll, also indicated Brown trailing Shaheen in a November showdown by 10 to 12 percentage points.

Brown, then a little-known state senator, in Massachusetts, scored an upset victory in a special election in January 2010 to serve the final three years of the term of the late Democratic Sen. Ted Kennedy, who had died the previous summer. Brown lost his bid 2012 re-election bid for a full six-year term to Warren.

At the end of last year, Brown sold his home in the Bay State and moved his residency north to New Hampshire, where he owed a vacation home and where he spent much of his childhood. In April he formally launched his Senate bid in the Granite State.