BOSTON – When the Tea Party Express pulls into the city where the conservative movement got its name, the crowd will be as notable for who is not there as who is.

Sarah Palin is the keynote speaker at today’s rally on Boston Common, but Republican Sen. Scott Brown – whose January election the movement claims as its proudest accomplishment – is skipping the event.

Officially, he’s too busy with his congressional duties – but Brown also kept the movement at a respectful distance during his upset campaign to succeed the late Democrat Edward M. Kennedy.

If he gets too close, the freshman senator, who’s still getting used to his national profile, risks being aligned with the tea party’s more radical elements, which have questioned the legitimacy of everything from President Barack Obama’s U.S. birthplace to his college degree.

“His ‘business in Congress’ is getting re-elected in 2012, and to do that, he needs to present a moderate image. Going to a tea party rally is about the last thing he needs,” said Jeffrey Berry, a political science professor at Brown’s alma mater, Tufts University.

“Brown doesn’t want to turn his back on his potential supporters, but he doesn’t want any photographs in the midst of an overly enthusiastic or bombastic event,” the professor added.

Even one of those who is going, independent gubernatorial candidate Timothy Cahill, a former Democrat, was careful to parse the meaning behind his attendance.

“He’s in campaign mode. He’s going to go where there’s mass groups of people,” said Cahill spokeswoman Amy Birmingham.

The rally, being held in the shadow of the Statehouse on Boston Common, is forecast to attract 10,000 people. It will be the next-to-last event in the 20-day, 47-city Tea Party Express tour concluding Thursday in Washington.