Party leaders said they would launch a statewide voter-registration drive on primary day, April 22. The intensely competitive race for the Democratic presidential nomination has prompted many Republicans to switch parties.

Republicans will coordinate a grassroots voter signup effort with county chairmen, and the GOP has been been negotiating with a firm that would pursue cases of fraudulent Democratic registrations. The state GOP will provide lists of party-switchers for volunteers to contact, and officials said they expected to bring two out of five defectors back into the Republican fold.

"We do want them back," said John McNally, who is in charge of the initiative for the Republican State Committee. "I want to assure every Republican that our party plans to address this issue."

As of Sunday, Democratic registration stood at 4.2 million, up nearly 8 percent since the November election. The state had 3.2 million registered Republicans, a drop of nearly 2 percent from the fall. Voters not affiliated with either party numbered 943,000, a reduction of just more than 4 percent.

Republicans said most of their decline consisted of people switching to vote in the Democratic presidential primary against Barack Obama or Hillary Rodham Clinton, rather than for one of them.

Abe Amoros, a spokesman for the Pennsylvania Democratic Party, said the Pennsylvania electorate had begun to move away from the Republican Party long before anyone knew that one party's presidential nomination would remain hotly contested.

Arizona Sen. John McCain is the presumptive Republican nominee.

"They have not hit the bottom depths of their valley just yet in terms of voter response," Amoros said. "This is the will of the people. That's why this is such a positive trend."

State GOP chairman Robert A. Gleason Jr. said a minor element of his party's strategy would be to go after people who are "double registered," particularly those who would be positioned illegally to vote in New Jersey and Philadelphia.

Amoros said that was a red herring.

"They've been saying this for years, and yet this claim of rampant abuse and fraud is never substantiated," he said.