Sen. Jeanne ShaheenCynthia (Jeanne) Jeanne ShaheenDem senator: If Nielsen doesn't reunite families, 'she should resign' America will not forget about Pastor Andrew Brunson Shaheen sidelined after skin surgery MORE (D-N.H.) posted her best fundraising quarter of the cycle with the specter of a Scott Brown challenge looming over her, raising more than $1.5 million in the first three months of the year.

That sum reflects donations from nearly 27,000 contributors, an increase of more than 12,000 from the previous quarter. Ninety-six percent of the donations were in sums of $250 or less, which campaigns typically see as an indication of strong grassroots support.

She now has $4.35 million cash on hand, according to her campaign.

But Shaheen is likely in the reelection fight of her life, with Brown officially kicking off his New Hampshire Senate campaign on Thursday night in Portsmouth, N.H.

The former Massachusetts senator, who moved to New Hampshire late last year and launched an exploratory bid for the race in early March, is a prolific fundraiser and is expected to draw millions both to his own campaign account and in spending from outside GOP groups supportive of his bid.

But he appears to be a money boon for Shaheen as well, who sent out a number of fundraising pleas throughout the past few months warning her supporters of Brown’s potential challenge.

Republicans believe Shaheen, though still personally well-liked in New Hampshire, is vulnerable because of her support for ObamaCare and the president -- both are unpopular in the state.

Brown has made those two issues central planks of his fledgling campaign against her, and plans to hammer her again on them, and tout himself as an independent, in his kickoff speech Thursday night.

"I worked with Senator Shaheen in the U.S. Senate for three years. She is a nice person, but wrong on the issues facing the people of New Hampshire. She made that clear when she cast the deciding vote that forced ObamaCare on this state and our country,” he’ll say, according to his prepared remarks.

Brown goes on to add that while “a lot of people aren’t aware” of Shaheen’s vote, “it’s important to know if we are ever going to get past ObamaCare and get America moving in the right direction.”

“I am running to be a true independent voice for New Hampshire – I am running to hold Senator Shaheen accountable,” he’ll say.

In the speech, Brown will also attempt to tackle the potential issue of his move to New Hampshire, which his detractors see as an opportunistic political move that shows he’s more concerned about his own political success than the state he uses to achieve it.

Brown has made efforts since launching his exploratory committee to emphasize his commitment and connections to New Hampshire, where he spent time with his grandparents while growing up and has owned a vacation home.

During his Thursday night speech, Brown will tout stops he’s made on his statewide listening tour over the past month, including “maybe a few pubs in between,” and his roots in the state.

And he’ll tell the crowd: “This is a state where people like to do their own thinking. They want to meet the candidates, look them in the eye, and ask their position on an issue. Sure, there are the usual political differences in New Hampshire. But across this state – wherever you go – you’ll find men and women with a pretty strong independent streak.”