Citing the backing of President Bush, Sen. Arlen Specter (search) voted Tuesday in the Republican primary, seeking a fifth term and hoping to ward off a strong challenge by Rep. Pat Toomey (search).

Polls opened at 7 a.m. and close at 8 p.m.

Specter has been a moderate mainstay in the U.S. Senate. But Toomey, a conservative, has moved to within striking distance of an upset win, according to recent polls.

"I think it's very important to focus on what President Bush wants," Specter said after he voted early Tuesday in Philadelphia's East Falls section. "He's the leader of the party. He thinks I can help him be re-elected."

Toomey, who on Monday said "it's time for a change," planned to vote later in the day in Zionsville in Lehigh County.

The race is a national litmus test of the strength of the conservative wing of the Republican Party. A Specter loss would throw the Republican's slim 51-seat majority in the Senate in doubt and could crimp President Bush's chances of winning much-needed moderate voters in the swing state this fall.

"The Democrats are on the sidelines hoping that I lose," Specter said.

As recently as last month, few believed Toomey, a junior congressman from the Lehigh Valley, could retire the state's senior senator. But a poll released on the primary's eve showed Specter with a slight 6-point lead over Toomey, and below the crucial 50 percent threshold.

Specter, 74, hopscotched across the state on Monday with fellow Sen. Rick Santorum. He called himself "cautiously confident," adding, "You don't see any perspiration on my brow."

But the 42-year-old Toomey, flying himself around on his own statewide tour, predicted an upset victory "whether we have a high turnout or a low turnout."

"I think we've done a good job getting our message out," he said. "I think people understand that it's time for a change."

The winner of the primary will face Democrat Rep. Joe Hoeffel, a three-term congressman from the Philadelphia suburbs, in the Nov. 2 election.

The Quinnipiac University poll released Monday showed Specter leading Toomey, 48 percent to 42 percent among 617 likely GOP primary voters. Ten percent remained undecided, according to the poll conducted April 20-24. It reported a sampling-error margin of plus or minus 4 percentage points.