Encouraged by a flurry of last-minute advertising and get-out-the-vote efforts, Sen. Arlen Specter’s (R-Pa.) backers are increasingly confident the four-term incumbent will pull out a win today over primary challenger Rep. Pat Toomey.

But they are taking nothing for granted in a race still expected to be decided by a handful of percentage points.

Supporters of both men privately concede that either candidate could win, with less than a third of registered Republicans expected to go to the polls today.

“We knew this race was going to come down to turnout,” said National Republican Senatorial Committee spokesman Dan Allen. “We feel confident in getting our voters out and we know that that’s going to be the determining factor in this race.”

In the final weekend of the campaign, Specter, 74, trained his turnout efforts on his base in the Philadelphia suburbs. He campaigned there Sunday with former New York City Mayor Rudy Giuliani, who reiterated Specter’s mantra that his victory in the primary would give President Bush a better chance of winning Pennsylvania in November.

Specter also brandished his endorsement from the National Rifle Association with an appearance at a gun show in Northeast Philadelphia Sunday morning.

Still, Toomey has questioned the ability of the 24-year incumbent, who has ruffled the feathers of many in his own party over the years despite their current support of his campaign, to turn out the GOP faithful.

Toomey supporters are “just itching to go vote,” according to Toomey campaign consultant Jon Lerner, while Specter has yet to demonstrate an ability to inspire such loyalty.

“I don’t think Senator Specter’s strength is any grassroots network,” Toomey said last week in a conference call with reporters. “There’s just no enthusiasm for him. There’s no energy for his campaign. There’s no passion. There are not armies of people who want to work hard to get him re-elected and that’s going to be a problem for him on Election Day.”

Toomey, a three-term conservative lawmaker from the Lehigh Valley, has defied both conventional wisdom and the naysayers who initially questioned his ability to compete with the entrenched incumbent.

Polls last week showed the race had narrowed to just 5 points, with Specter still holding the edge in the nationally watched contest dubbed the fight for the soul of the Republican Party.

A new Quinnipiac University survey released Monday showed the margin between the two men at 6 points, with Specter still below the critical 50 percent mark.

Buoyed by a last-minute visit from Bush, and new ads touting his endorsement of Specter, the Senator’s backers argued Monday that Toomey’s apparent momentum last week had been diffused before the final hours of campaigning.

“I do think whatever momentum Toomey had going into the weekend is not there now,” said one Republican close to the Specter campaign, before adding the caveat, “We’re by no means out of woods.”

Toomey’s campaign, meanwhile, continued to exude confidence in its candidate’s ability to win.

“I’ve not seen any evidence that the momentum has stopped moving,” said Lerner. “There’s a huge amount of excitement on the ground there.”

Toomey, in a rare display of precise confidence, predicted in the Thursday conference call that he would win with 51.7 percent of the vote, noting that undecided voters traditionally break heavily toward the challenger on Election Day.

“I think it’s going to be a close race, as that is,” Toomey said. “But I don’t think it’s going to be very late into the night before the outcome is clear.”

But that confidence, coupled with last week’s polling numbers, has helped to motivate Specter supporters, his backers now contend.

“I don’t think there was a realization on a lot of the Specter voters that this race was so tight that he could really lose” until last week, said the source close to Specter’s campaign.

Specter and Toomey spent Monday zigzagging the state by air.

Specter, campaigning alongside fellow GOP Sen. Rick Santorum (Pa.), began the day in Pittsburgh, then flew to Johnstown, Harrisburg, Wilkes-Barre, Erie and Philadelphia.

Toomey, piloting his own plane, made stops to rally supporters in Erie, Scranton, Harrisburg, Johnstown, Pittsburgh and then Allentown, his home base.

Specter has been using footage from an appearance last Monday with Bush in two television ads that began airing late last week. One of the ads was partially funded by the National Republican Senatorial Committee.

Specter has spent upward of $12 million on the race, while Toomey has spent close to $4 million. The Club for Growth, which is backing Toomey, has spent more than $1.4 million on advertising in the race through its political action committee.

Meanwhile, Rep. Joe Hoeffel, who will become the official Democratic Senate nominee on Tuesday, will launch his “A Future That Works” tour Wednesday, visiting 19 counties in seven days.

Democratic hopes of picking up the Senate seat in Pennsylvania have been buoyed by Toomey’s rise in the polls, and they have done little to mask their desire for a matchup with the Congressman rather than with Specter in November.

While Hoeffel has faced a non-competitive primary in the Senate race, the 13th district contest to replace him has featured bitterly contested battles on both sides.

On the Democratic side, state Sen. Allyson Schwartz and former National Constitution Center President Joe Torsella are the leading contenders.

Schwartz, a liberal state lawmaker who lost a Senate primary in 2000, has been aided heavily by the backing of EMILY’s List, which is also helping her campaign with turnout efforts.

Torsella, a former aide to now-Gov. Ed Rendell (D), scored the endorsements of three of the district’s two major newspapers, the Philadelphia Daily News and The Philadelphia Inquirer and the Northeast Times.

On the Republican side, wealthy ophthalmologist Melissa Brown and state Rep. Ellen Bard are locked in a tough fight for their party’s nomination.

Brown, who has run for the seat twice before, was given the early advantage in the race. But after Bard narrowly won the key endorsement of the Montgomery County GOP, the bitter and sometimes ugly contest essentially became a tossup.

Both parties expect a competitive, expensive general election race in the 13th, which is comprised of almost even portions of Northeast Philadelphia and its Montgomery County suburbs. The swing seat was made slightly more Democratic during redistricting.

Meanwhile, in the 15th district race to succeed Toomey, state Sen. Charlie Dent (R) and attorney Joe Driscoll (D) are expected to win their respective nominations, although both face primary challenges.

The swing district voted 49 percent to 48 percent for then-Vice President Al Gore in the 2000 presidential contest.

Republicans are also monitoring the outcome of the 17th district GOP primary, where several candidates are vying for the chance to face Rep. Tim Holden (D) on the November ballot.

No public polling has been released in the clouded contest, but the leading contenders are attorney Scott Paterno, accountant Frank Ryan, former state Adjutant General Bill Lynch and attorney Mark Stewart.

Paterno, the son of Penn State football coach Joe Paterno, is considered the nominal frontrunner.

In the only other primary activity of note Tuesday, Rep. Bill Shuster (R) is facing off against political newcomer Michael DelGrosso, in what has been a bitter fight between two of the area’s best-known names.

DelGrosso’s family operates DelGrosso’s Amusement Park in Tipton, Pa., and DelGrosso Goods Inc., which produces 1.5 million cases of tomato sauce annually.