The worst-kept secret in midterm politics has finally passed its expiration date. Former US Senator Scott Brown, who represented Massachusetts after Ted Kennedy’s death, will run against New Hampshire incumbent Jeanne Shaheen in November, assuming he wins the GOP primary in the Granite State. Brown officially opened his campaign with a blast against ObamaCare, which he said forces New Hampshire voters to “live free or log on” rather than operate under the state motto of “live free or die”:

He accused Democratic incumbent Jeanne Shaheen of being a rubber stamp for President Barack Obama, and criticized her support for the federal health care overhaul law.

“It forces us to make a choice – live free or log on.  Right? Live free or log on.  And guess what?  Guess what? In New Hampshire, guess what we choose? We choose freedom,” Brown said.

Brown said if he goes back to Washington, he won’t be anyone’s yes man.

“We don’t just follow the crowd wherever it’s going. In government, especially in government, we expect more of ourselves than just to follow a party boss or a party line,” Brown said.

Brown expects to get attacked over his move from Massachusetts to New Hampshire, although he told WMUR that voters don’t mention it to him at all. Both Shaheen and other Republican primary opponents have taken aim at carpetbagging. Brown’s campaign tried reversing that on Shaheen at yesterday’s event:

Brown was introduced by former Gov. John H. Sununu, who called Shaheen Senate Majority Leader Harry Reid’s “favorite rubber stamp. She also happens to be Barack Obama’s favorite rubber stamp.”

“This is the third senator from Massachusetts.”

Even though Brown hadn’t officially entered the race, Shaheen’s campaign has been going after him in an attempt to either pre-empt his attacks or perhaps persuade him to sit out the midterms. A new Granite State poll from WMUR shows that it’s not working very well:

A new poll shows that as Republican Scott Brown prepares to officially announce he’s running for Senate in New Hampshire, he has gained some ground on U.S. Sen. Jeanne Shaheen.

The WMUR Granite State Poll shows that Shaheen leads Brown 45 to 39 percent, with 14 percent undecided. In a January poll, Shaheen had a 10-point lead, 47 to 37 percent.

The poll of likely New Hampshire voters was conducted by the University of New Hampshire Survey Center by landline and cellphone from April 1 through April 9 and has a margin of sampling error of plus or minus 5 percentage points.

That’s not much of a gain, but the attempts by Shaheen to marginalize Brown over the last three months aren’t having any impact. At 45% and declining (slightly), Shaheen looks vulnerable — especially since the GOP hasn’t even selected its nominee yet. Brown has to pick up his favorability numbers, but he’s got plenty of upside in this poll and a change to change the unfavorability figure with a New Hampshire-focused campaign.