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 Kennedy.

If he gets too close, the freshman senatorrisks 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," Berry said.

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.

Palin spoke on the first day in Searchlight, Nev., hometown of Senate Majority Leader Harry Reid, a Democratic target of the movement.

Her visit to Boston brings her to the site of the original Tea Party in 1773, where British opponents rallied against taxation without representation.

Two others are embracing the modern tea party movement, which questions the Washington establishment, without reservation.

Conservative talk show host Michael Reagan, son of Republican icon Ronald Reagan, spoke Tuesday at a tea party rally in Jefferson City, Mo.

He told a crowd of about 400 that liberals and progressives are like "termites who eat away at the foundation" of freedom in the United States by imposing more and more government control.

Nebraska Gov. Dave Heineman was headlining at a tea party rally Tuesday night in his own capital city, Lincoln. Tea partiers also rallied Tuesday in Albany, N.Y.

A Brown spokesman said the senator will be in Washington today, attending to the business for which he was elected. Congress just returned after a two-week recess, during which Brown traveled to Afghanistan and toured flood-stricken areas of Massachusetts.

"While he is unable to attend Wednesday's event, the senator appreciates the strong grassroots support he received from a wide range of individuals, including those who are part of the tea party movement. He hopes they have a successful event," spokesman Colin Reed said in a statement.

Brown spoke a year ago at two tea party Tax Day protests in Massachusetts, and his campaign hosted a breakfast in January for local tea party activists.