WASHINGTON - An international labor union is urging its Democratic members in Pennsylvania to switch their voter registration to Republican to vote for Sen. Arlen Specter in his tough primary fight against conservative Rep. Pat Toomey.

The registration push comes as national groups scramble to sign up voters for November's presidential race in Pennsylvania, a battleground state and the nation's fifth-largest electoral prize. Voters who don't switch back risk falling through the cracks during Democratic get-out-the-vote drives and other outreach efforts.

Specter, a political moderate who generally supports labor issues, is the only sitting senator in the nation to face a primary. He "needs as much support in the April 27 primary as possible," wrote Robert A. Scardelletti, president of the Transportation Communications International Union in a March 15 letter to Pennsylvania members.

"Enclosed is a voter registration card that you can use to register to vote in the Republican primary if you so choose," Scardelletti wrote in the letter, a copy of which was obtained by The Associated Press. "I realize that this is a somewhat unusual request, but I can assure you that it is vitally important.

While labor unions generally support Democrats, Scardelletti's letter was accompanied by instructions from the Specter campaign on how to register to vote. Scardelletti did not immediately return a phone call seeking comment Wednesday.

Registered Democrats outnumber Republicans by about 445,000 voters in Pennsylvania. The state's primary voter registration deadline is March 29.

Pennsylvania, a political swing state that swung for Democrat Al Gore by a mere 204,000 votes in 2000, is on the national forefront of an intense push this year by both parties to recruit voters. Parties use registration rolls to target voters with campaign fliers and, in turn, push them to the polls.

Since many union members are expected to support Democratic presumptive presidential nominee John Kerry in November, "it's probably not necessary for them to switch back," said Norman Adler, a New York-based political consultant who advises unions.

Even so, "it's very hard" for union leaders to persuade their members to go through yet another registration switch so soon after the first, Adler said. He called the union's push to switch voter registration highly unusual in a U.S. Senate race.

Aides to Toomey, the conservative challenger, said the union support indicates a "late, frantic appeal" to Democrats by Specter.

"The fact that this appeal comes just days before the registration deadline illustrates that Arlen Specter is aware that he doesn't have sufficient support among Republicans to win this race," Toomey campaign spokesman Joe Sterns said.

But Specter campaign manager Christopher Nicholas said Specter would work with any group to try to boost the number of Republican voters in the state to help President Bush in November.

"And we are working with any group that wants to work with us," Nicholas said.