A surge in support for conservative challenger Rep. Pat Toomey (search) has narrowed Sen. Arlen Specter's (search) lead to a scant 5 percentage points a week before Pennsylvania's Republican primary, according to a poll released Tuesday.

Toomey, a three-term congressman from the Allentown area, was backed by 44 percent of likely Republican voters, compared with 49 percent for Specter in the survey by the Quinnipiac University Polling Institute (search). Seven percent were undecided.

In a Quinnipiac poll released April 7, Specter held a comfortable lead of 15 percentage points. The primary is April 27.

Fifty-one percent of the respondents in the latest poll said they considered the moderate Specter, who is seeking a fifth term, "too liberal," up from 43 percent in the previous survey. Only 14 percent said they consider Toomey too conservative.

Clay Richards, assistant director of the Connecticut-based polling institute, said the swing in support for Toomey came from conservative defectors from Specter's camp.

"Toomey has turned his challenge into a real horse race, and the outcome depends on whether Specter can halt the conservative defections and rally his own liberal and moderate Republican base," Richards said.

Political observers are closely following the Pennsylvania race as the Republicans' 51-seat majority in the Senate is put to the test this year.

Toomey's candidacy is seen as the strongest challenge to a sitting senator, and the primary contest is seen by conservatives as a litmus test of the strength of the GOP's right wing.

President Bush has endorsed Specter and appeared at a Specter fund raiser in Pittsburgh on Monday night. But national conservatives have funneled nearly $2 million worth of contributions and attack ads into the state to help Toomey, who has also spent another $2 million since the start of the year.

Specter, who has fought back with more than a dozen of his own ads, had spent $7 million — half his war chest — between Jan. 1 and April 7.

The winner of the primary will face Democratic Rep. Joe Hoeffel, a three-term lawmaker from suburban Philadelphia, in the Nov. 2 general election. Both Hoeffel and Specter have said the Senate seat will most likely go to the Democrats if the conservative Toomey is the Republican nominee.

Among people who identify themselves as conservatives, whom Toomey has sought to court, 56 percent said they support Toomey and 39 percent were for Specter.

The poll carries a sampling-error margin of plus or minus 5 percentage points.