President Obama will travel to Massachusetts on Sunday to campaign for Martha Coakley, according to sources.

The trip represents an unprecedented role for Obama in a special election. He didn't appear during the runoff in Georgia's Senate race in December 2008 or in an early 2009 New York special House election.

He did hold a fundraiser for now-Rep. Bill Owens (D-N.Y.) in another special House election in New York late last year, though it was outside the district, in New York City, and included no public events.

White House press secretary Robert Gibbs on Friday stressed that the race was important, but that it was not a referendum on the Obama White House. 

"It's an important Senate seat. That is why the president is going," Gibbs said at his daily press briefing Friday.

But Gibbs said "We're not on the ballot" when asked if it was a referendum on the administration. 

Gibbs said that the Coakley campaign just invited Obama to make a campaign stop on Friday, even though speculation about an Obama appearance whirled around all this week.

Obama has recorded a web video and robocalls for Coakley, who finds herself in an unexpected dogfight with state Sen. Scott Brown (R) in the race for Sen. Ted Kennedy's (D-Mass.) seat.

Jordan Fabian contributed to this report