If former Massachusetts Sen. Scott Brown has in recent weeks elevated the profile of the New Hampshire Senate race while moving toward a Republican bid, he has also helped bring Democratic donors' attention -- and money -- to incumbent Sen. Jeanne Shaheen, D-N.H.

Shaheen's campaign will report Thursday having raised more than $1.5 million from January through the end of March, finishing last month with $4.35 million on hand. The campaign's donor pool expanded by more than 12,000 people from the previous quarter, drawing money from more than 26,000 donors -- most of whom gave less than $250.

"Our campaign is proud of all the grassroots support our campaign has generated to date and we're confident we'll have the resources we'll need to win this November, regardless of whom Republicans nominate to run against Jeanne Shaheen," said Harrell Kirstein, a spokesperson for Shaheen.

Brown is expected to advance with relative ease from the Republican primary to challenge Shaheen in the general election. The campaign will begin for Brown in earnest Thursday evening when he officially launches his Senate bid in Portsmouth, N.H., capping off a few weeks of barnstorming the state to build enthusiasm for his candidacy.

The race will likely be hard-fought between Shaheen, a well-liked former governor in her first Senate term, and Brown, who lost his re-election bid to Democratic Sen. Elizabeth Warren in 2012 in Massachusetts and moved to New Hampshire full-time late last year.

In Brown's previous race, he and Warren agreed to a "People's Pledge" to limit outside money in the campaign. Shaheen's campaign urged Brown to honor the same agreement this year, but to no avail.

Instead, the race promises to be among the more expensive ones of the midterm election cycle. Some New Hampshire political operatives estimate the candidates alone could spend between $8 million and $10 million by Election Day, in addition to the significant money expected to flow from third-party groups on both sides.

Outside groups including American Crossroads, a pro-Republican organization founded by Karl Rove, and the pro-Democratic Senate Majority PAC have already begun to air television advertisements in the state.

Heavy-hitting political surrogates are stepping up on both sides, too. Warren has helped to raise money for Shaheen, and former New York Mayor Rudy Giuliani plans to campaign for Brown in New Hampshire later this month.