BOSTON -- With a little-known Republican surging in the polls, threatening an upset of what was thought to be the Democrat's shoo-in replacement for Massachusetts Sen. Ted Kennedy in the U.S. Senate, Democrats are pulling in the big guns, bringing President Barack Obama to campaign on Sunday.

The president's visit will follow a Clinton campaign swing through the Hub for Coakley on Friday.

Obama is coming to lend support to state Attorney General Martha Coakley, who is running against State Sen. Scott Brown and Independent candidate Joseph L. Kennedy (no relation to Ted Kennedy) for the coveted seat.

Kennedy died of brain cancer last August.

Recent polls show the newcomer Brown making a strong last-minute surge, coming close to a statistical tie with the well-financed, well-known attorney general.

To bolster Brown's chances, former New York Mayor Rudolph Giuliani arrived in the Hub Friday, meeting with Brown in the shadow of the Old North Church in Boston's North End.

Giuliani told supporters that electing Scott Brown to the seat held by Kennedy for nearly 50 years would send a loud message to Washington that voters believe the nation is going in the wrong direction.

"(Brown's) election, I believe will send a signal -- and a very dramatic one -- that we're going in the wrong direction on terrorism. And we have to change it, and we have to change it now," Giuliani said.

Giuliani came to Boston just as a Suffolk University poll showed Brown leading Democratic opponent Martha Coakley by 4 points -- 50 percent to 46 percent.

The latest poll numbers could reshape the candidates' approach to the final weekend of campaigning in an election that the Democrats expected to easily win in this traditionally Democratic state, where independent voters now outnumber members of either established party.

A possible Brown win is a threat to the Democratic agenda because it would mean the loss of their 60-vote Senate supermajority. Obama has been counting on that to prevent Republican filibusters so he can pass health care reform and other measures.

President Bill Clinton also be in Boston Friday to support Coakley.

"I'm here today because I believe in Martha Coakley. I believe in her potential as a United States senator. I believe in the record she has established as attorney general. And I believe in the choice that she presents against her opponent. And I know what is going on because I have seen this movie before, and so have you," Clinton said.

Kennedy died in August of brain cancer after a nearly 47-year career. Independent candidate Joseph L. Kennedy -- no relation to the late senator -- is also on the ballot Tuesday.