The Republican primary race for the U.S. Senate in Pennsylvania remained tight, a new poll showed Monday, as moderate Sen. Arlen Specter (search) and conservative Rep. Pat Toomey (search) campaigned across the state on the last day before the election.

Specter held a slim margin over Toomey, 48 percent to 42 percent, according to the Quinnipiac University survey of 617 likely GOP primary voters. Ten percent of voters remained undecided. The poll, conducted April 20, has a sampling-error margin of plus or minus 4 percentage points.

The previous Quinnipiac survey, released last week, showed Specter with a 5-point lead over Toomey, 49 percent to 44 percent. The error-margin was plus or minus 4.5 percentage points.

"There was very little movement in the numbers this last week," said Clay F. Richards, assistant director of the Quinnipiac University Polling Institute. Specter "holds a slight lead, but it's a close race and anything could happen."

"It comes down to which candidate does the better job of getting his voters out tomorrow," Richards said.

The election caps one of the most closely watched Republican fights in the country this year, and one that could chance the political face of the slim Republican majority in the Senate.

Both candidates set off on lengthy campaign swings Monday with nearly overlapping routes ranging from Pittsburgh and Erie in the west to Wilkes-Barre and Scranton in the east.

The winner of the GOP primary will face three-term Rep. Joe Hoeffel in the November general election; Democratic voters narrowly outnumber Republicans statewide.

Toomey, claiming strong statewide support, said he hoped for a high voter turnout to give him a decisive win. But most political analysts agree that a low turnout will help the congressman because a higher turnout would bring greater numbers of moderates who support Specter.

More than 3.1 million voters are registered as Republicans in Pennsylvania. Ten percent to 25 percent of GOP voters turned out in Specter's 1998 and 1992 primaries.

"So much is going to depend on turnout, and turnout is hard to predict," said Villanova University political scientist Robert Maranto. "And that's what makes this an exciting race."