Brown to formally jump into Senate race

(CNN) - Scott Brown is about to make it official.

The former senator from Massachusetts, who's been moving closer and closer to running for the Senate in neighboring New Hampshire, formally tosses his hat into the ring at a campaign event in Portsmouth, New Hampshire on Thursday, CNN confirmed Monday morning.

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The sea coast city setting for Brown's official entry into what may become a major battle against incumbent Democratic Sen. Jeanne Shaheen, emphasizes Brown's roots in the Granite State. His mother lives in the vicinity, he spent a lot of his childhood in the area, and he was born just across the Piscataqua River, in Kittery, Maine. . Brown's announcement was first reported by John DiStaso of the New Hampshire Journal.

Since late year, when he began considering a Senate bid in New Hampshire, Brown's been highlighting his ties to the Granite State, to push back against characterizations of him as a "carpetbagger" by Democrats.

"Portsmouth played a special role in my childhood. I remember our house on Islington Street, strolling through Strawbery Banke with my grandfather, and catching a show at Prescott Park," said Brown in an email to supporters. "Now, Portsmouth is going to be the start of the next chapter in my life."

Last month, Brown announced that he was forming an exploratory committee, which allowed him to raise money and hire staff for a Senate bid. He immediately began a listening tour across New Hampshire. And last Friday CNN confirmed that a former longtime close aide, Colin Reed, had left his job as deputy communications director for New Jersey Gov. Chris Christie to serve as Brown's campaign manager.

Brown has made his opposition to the federal health care law a key part of his campaign. In his email, Brown added that as he's traveled across the state, "I’ve heard the same concerns at every stop. You’ve told me you want a stronger economy with more good-paying jobs. You want a government that doesn’t spend more than it takes in. Most of all, you want a health care system that works for New Hampshire – not one that leaves you with higher premiums, cancelled policies and fewer medical options."

From Massachusetts to New Hampshire

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 Democratic challenger Elizabeth Warren.

Last year, Brown passed on running in a special election in Massachusetts to fill the term of John Kerry, who left the Senate to become secretary of state. And at the time, Brown also announced that he wouldn't make a 2014 bid for an open governor's seat in the Bay State.

Brown made a number of speaking appearances at GOP events in New Hampshire last year, where he spent much of his childhood and where he owned a vacation home. Last fall, in another hint about a possible run, he dropped the 'MA' from his Twitter handle.

A few months later, he sold his home in Massachusetts and moved his residency north to the Granite State.

Battle over a pledge

Shaheen's campaign, reacting to the news that Brown is formally entering the contest, quickly criticized him for not signing a pledge to keep outside money out of the Senate race.

"In 2012, Scott Brown said Massachusetts voters deserved better than outside third-party attack ads. Well, the people of New Hampshire deserve better too. We hope, now that he is officially a candidate, Scott Brown will commit to the same standard he set in his last race on the other side of the border and tell the special interests to stay out," said Mike Vlacich, Shaheen's campaign manager.

Both Brown and Warren signed the pledge in their 2012 contest, but he won't sign the new pledge, saying it comes too late because outside groups have already spent big bucks on the race.

One of those groups, the Ending Spending Action Fund, a pro-Republican outside group, says its going up statewide in New Hampshire with a new TV commercial that uses a clip of Brown from four years ago slamming the new federal health care law, better known as Obamacare. The spot ends with the narrator saying that "Scott Brown was right on Obamacare then, he's right for New Hampshire Now.

The group tells CNN that they're spending six figures to run the spot starting Monday on WMUR, the main commercial broadcast station in the Granite State, as on NESN (New England Sports Network) during Boston Red Sox pre and post game shows, including Thursday's first showdown of the year between the Red Sox and the rival New York Yankees

The group put out a spot last year in an effort to convince Brown to run for the Senate in New Hampshire.

Separately, two of the biggest players when it comes to outside spending, Americans for Prosperity (which is backed by the big bucks of the billionaire industrialist Koch Brothers) and American Crossroads (which was co-founded by Karl Rove), have poured around $1.5 million into the race, mostly to attack Shaheen for her support of the health care law.

"Scott Brown is for Scott Brown. He moved to New Hampshire and aligned himself with the third party special interest groups trying to buy New Hampshire's Senate seat," said Granite State Democratic Party Chairman Ray Buckley, in a statement.

"Today’s announcement is no surprise-Scott Brown is working with the Koch Brothers and Karl Rove to get elected in New Hampshire so that he can do their bidding again in the United States Senate. Well, Granite Staters know and deserve better," Buckley added.

State of Play

Brown joins three other Republicans who are running for their party's Senate nomination in New Hampshire: former U.S. Sen. Bob Smith, former state Sen. Jim Rubens, and conservative activist Karen Testerman. The winner will face off against Shaheen, also served six years as the state's governor.

If Brown ends up winning the GOP's September primary, it could expand the map for Republicans. Democrats hold a 55-45 majority in the Senate (53 Democrats and two independents who caucus with the party), but are defending 21 of the 36 seats up in November, with half of those Democratic-held seats in red or purple states, like New Hampshire.

The most recent public opinion polls in a possible Brown-Shaheen November showdown give the incumbent a double digit advantage. According to a Suffolk University/Boston Herald poll conducted in March, Shaheen had a 52%-39% lead among New Hampshire voters. And the senator was up 50%-38% over her potential GOP challenger in an American Research Group survey conducted around the same time.