Gabriel Gomez, right, the Republican candidate for U.S. Senate in the Massachusetts open seat special election, points something out to former New York City Mayor Rudy Giuliani during a walk on Boylston Street in Boston, Thursday, June 6, 2013. Gabriel Gomez, right center, the Republican candidate for U.S. Senate in the Massachusetts open seat special election, walks with former New York City Mayor Rudy Giuliani to greet potential voters during a walk on Boylston Street in Boston, Thursday, June 6, 2013. U.S. Senate candidate Democrat Edward Markey gestures during a debate, Wednesday, June 5, 2013 in Brighton, Mass. Democrat Edward Markey and Republican Gabriel Gomez have clashed in their first debate in Massachusetts special U.S. Senate election, with each candidate trying to portray the other as out of step with ordinary citizens. AP Photo/The Boston Globe, Yoon S. Byun, Pool U.S. Senate candidate Republican Gabriel Gomez, left, shakes hands with Democrat Edward Markey before the debate, Wednesday, June 5, 2013 in Brighton, Mass. BOSTON (AP) - Democrats are dispatching a lineup of political heavyweights to Massachusetts, backed by a river of outside money, to head off the possibility that another upstart Republican pulls off a Senate special election stunner. National Republican groups have been reluctant to devote resources to a race that many Washington-based strategists have thought unwinnable for the GOP. Yet both parties know special elections draw far fewer voters - and they remember the special election in 2010 that ended with a Republican winning the Senate seat long-held by the late Edward M. Kennedy. And Democrats, already down one Senate seat with the death this week of Sen. Frank Lautenberg of New Jersey, don't want to appear complacent even as polls suggest a likely victory when voters cast ballots in two weeks. In support of the party's nominee, longtime Rep. Ed Markey, Democratic National Committee chair Debbie Wasserman Schultz is visiting Massachusetts this weekend. On Tuesday, two vice presidents - Joe Biden and Al Gore - are set to raise cash for Markey in Washington. President Barack Obama, Democrats' most powerful political weapon in a state that overwhelmingly backed him last fall, campaigns with the candidate a day later. Gabriel Gomez, the Republican running to replace former Sen. John Kerry, is casting Obama's visit as a sign of the GOP's own fortunes. "Congressman Markey must be feeling some extreme heat to bring in somebody of President Obama's caliber," says Gomez, a former Navy SEAL with no political experience. Gomez drew attention Thursday when former New York Mayor Rudy Giuliani joined him in downtown Boston, and Gomez plans to greet voters outside the TD Garden before the Boston Bruins playoff game Friday. Markey is pushing back with a campaign swing Friday alongside Rep. Xavier Becerra of California, chairman of the House Democratic Caucus. Until now, the special election has garnered little attention inside Massachusetts following the Boston Marathon bombings, much less outside the state. Republicans are waiting to see if the contest tightens enough to justify spending big on TV ads to try to repeat the shocker of 2010 when the Scott Brown unexpectedly won Kennedy's seat. Brown served less than a full term. He was defeated last year by Elizabeth Warren, a liberal Democrat and former Harvard Law School professor. Markey, the dean of the state's congressional delegation, has led every public and private poll released in recent weeks. He enjoys the inherent advantages of being a Democrat in a state where Democrats dramatically outnumber Republicans. But Republicans believe there's an outside chance that they can again eke out a victory over a Democrat in the liberal-leaning state. Both sides expect a flood of new advertising in the coming days. So far, Democrats have outspent Republicans roughly $2 million to $1.5 million, according to officials who track political advertising. The Senate Democrats' campaign arm has reserved another $750,000 for statewide television ads to help Markey, while the Democratic-aligned Senate Majority PAC planned to invest another $700,000 in the final weeks. The League of Conservation Voters also has pledged to spend $400,000 on mailings to benefit the Democratic nominee. Gomez said Friday he's unfazed by the spending. The decisions "to flood Massachusetts with more dirty, negative attacks prove that national Democrats are now in a full-fledged panic," he said. Gomez has relied on the Massachusetts Republican Party to help pay for his television ads. The spending disparity aside, the uncertainty of off-year special elections and the lessons of Brown's 2010 victory loom large for Republicans and Democrats alike. "Democrats are confident, but we're taking nothing for granted," said Matt Canter, the deputy executive director of Senate Democrats' national campaign arm. More than Democratic pride is at stake. Democrats narrowly control the Senate, and party officials acknowledge that Obama can't afford to lose another reliably liberal vote when he has big-ticket legislative priorities on his plate. Upcoming votes on immigration and the budget could come down to just a few votes. After Lautenberg's death, New Jersey Gov. Chris Christie tapped a fellow Republican to fill the seat until a special election in October. Few GOP stars or organizations have been willing to help Gomez, who acknowledged having donated to Obama during the 2008 election cycle. Associated Press writers Josh Lederman in Washington and Steve LeBlanc and Bob Salsberg in Boston contributed to this report.