NASHUA — Scott Brown, whose star rose fast and set quickly in Massachusetts, began charting what he hopes will be a course to redemption in a new state, declaring on Friday that he will begin building a campaign for the US Senate in New Hampshire.
His announcement at the Northeast Republican Leadership Conference marked a turning point in a months-long political flirtation that had begun to grate on New Hampshire Republicans, who were eager for Brown to declare his intentions.
Brown said he would launch an exploratory committee, a step that allows him to begin raising money and hiring staff in hopes of challenging Senator Jeanne Shaheen, a Democrat. He said he would start touring the state on Saturday.
“We look forward to meeting you and to this great journey ahead,” Brown said in a speech at the conference Friday afternoon, sparking enthusiastic applause in a room packed beyond capacity with Republican leaders and donors and national media.
The race is expected to be among the most competitive and expensive in the nation, since it is one of a handful that could determine control of the Senate. A prodigious fund-raiser and energetic campaigner, Brown had been heavily courted by Republican leaders who consider him the party’s best chance to take on Shaheen.
American Crossroads, the super PAC affiliated with Republican strategist Karl Rove, said Friday that it would launch $600,000 worth of attack ads against Shaheen, starting Tuesday. The bombardment is expected to last a week.
Fox News, where Brown had been a paid contributor since early 2013, said it would end its contract with the former senator. Nixon Peabody, the Boston law firm that Brown joined a year ago, said he would remain an attorney there.
Before Brown can challenge Shaheen in November, he must face three Republicans in a September primary: former US senator Bob Smith, former state senator Jim Rubens, and conservative activist Karen Testerman. But Brown is expected to be the most formidable candidate in that field.
Early polls suggested that a Brown-Shaheen race could be a toss-up, but more recent surveys indicate she has a sizable lead. A Suffolk University poll released last week found Brown would pose a strong challenge to Shaheen, but still trails her 52 percent to 39 percent, with 9 percent undecided.
In his speech Friday, Brown, 54, sought to establish his ties to New Hampshire and sketch the themes of his campaign.
He began on familiar turf by pointing proudly to the truck he drove across Massachusetts in a 2010 special election held after the death of Democrat Edward M. Kennedy, who had held the seat for 47 years. The truck, he said, now has 300,000 miles on it, and its Massachusetts license plate has been replaced by one from New Hampshire.
He fondly recalled that his parents lived in Portsmouth when he was born and that he spent summers with his grandparents exploring the rocky coast in Rye and visiting Hampton Beach.
“So much of my life played out in Massachusetts . . . but a big part of it was always right here in New Hampshire,” he said.
He also blasted the federal Affordable Care Act, seizing on an issue that helped catapult him into the Senate in 2010, when he used the issue to help stir up grass-roots support from Republicans and Tea Party activists. Polls show the law is unpopular in New Hampshire, and Republicans believe it will be a major liability for Shaheen, who voted for it.
Brown indicated that his desire to repeal the law would be a driving force behind his campaign.
“A big political wave is about to break in America and the Obamacare Democrats are on the wrong side of that wave,” Brown said. He argued that the health care system was “very good” before President Obama and his fellow Democrats pushed the federal overhaul.
Fergus Cullen, a former chairman of the New Hampshire GOP, said he felt that Brown’s speech “made a great first impression.” He was among those who had been urging Brown to make his plans clear.
“I think there’s a tremendous relief among establishment Republicans,” Cullen said. “They are very excited by the prospect of a Brown candidacy. Of course, there are going to some on the far right who are not excited about this, but they’re in the minority. For every detractor, there are 10 enthusiasts as of today.”
Some conference attendees, however, cautioned Friday that his name recognition would only take him so far in a state where voters are accustomed to being courted face-to-face by presidential candidates.
“I need more substantive conversations,” said James McConaha, an independent voter who traveled from Concord to hear Brown speak. “Today was kind of a pep rally, ‘rah-rah-rah’ thing.”
Matt Mowers, executive director of the New Hampshire GOP, argued that Brown would not be an automatic favorite in the Republican primary.
“New Hampshire is a traditional retail politics state,” he said. “The voters here want to hear you talk two or three times, not just once. People are much more interested in where you stand on issues and how you’ll help them.”
A national political star after he scored an improbable victory over Democrat Martha Coakley in 2010, Brown’s fortunes plummeted in 2012, when he was ousted by Democrat Elizabeth Warren. He moved to his summer home in Rye late last summer and registered to vote there in December.
Democrats on Friday wasted no time labeling him a carpetbagger. The New Hampshire Democratic Party released a video showing footage of Brown from his past campaigns boasting that he was a “Massachusetts Republican” who had grown up in the state and planned to remain there.
“Like other New Hampshire tourists we hope he feels welcome up here and spends some money, but he should know that New Hampshire’s North Country is Jeanne Shaheen country,” the party said in a statement.
The Shaheen campaign blasted out an e-mail seeking donations, saying, “It’s the only chance we have to defend the Senate against the avalanche of Big Oil, Big Bank, and other secret corporate cash headed our way.”
Warren sent her own e-mail Friday asking her supporters to donate to Shaheen and rally behind her colleague from New Hampshire as they did for her two years ago.
“Scott Brown may have moved to the Granite State, but we’re not going anywhere,” Warren wrote.
In a sign of how fluid Brown’s plans were until Friday, he began the day by canceling a planned trip to Iowa next month. That visit was widely seen as a sign that Brown harbored ambitions to run for the White House in 2016.
“He texted me this morning by 6 a.m., saying he’s going to run for the United States Senate in New Hampshire and won’t be able to come to Iowa,” said Tracee Knapp, a GOP activist in Ringgold County who had invited Brown to speak to a party gathering there on April 3.
“We’re pretty disappointed,” she said. “We’re trying to scramble to get someone else to speak.”
@globe.com. Follow him on Twitter @mlevenson. Dan Adams can be reached at email@example.com.