It's not just birds returning from the South who are migrating this spring.

The potential for next Tuesday's primary to decide the race between U.S. Sen. Hillary Clinton and U.S. Sen. Barack Obama for the Democratic presidential nomination no doubt is the main factor behind the Democratic Party's gain of more than 300,000 registered voters in Pennsylvania since the November election.

As of April 6, there were about 4.2 million registered Democrats and 3.2 million registered Republicans statewide, and the GOP has lost more than 50,000 voters statewide since November, according to the Pennsylvania Department of State.

• In Allegheny County, Democratic registration has increased by more than 25,000 and GOP registration has decreased by more than 2,000.

• In Westmoreland County, Democratic registration has increased by more than 4,400 and GOP registration has increased by about 190, but remains between 85,000 and 86,000.

State-level GOP leaders plan to have people at the polls next Tuesday who will try to urge voters who switched from Republican to Democrat to vote in the Clinton-Obama contest to return to the GOP fold for the November election.

Some of the newly registered Democrats are longtime Republicans who think they can help U.S. Sen. John McCain, presumptive GOP nominee, by helping nominate the Democrat they think would be easier for him to defeat. Don't expect those voters to remain registered Democrats in the long run.

But also among those newly registered Democrats are former GOP voters who are fed up enough with the Bush administration to be "on the fence" about how they'll vote in November.

And because party registration doesn't keep Pennsylvanians from voting in any races in general elections, ex-GOP voters newly registered as Democrats lack a strong incentive to bother with switching again before November.

Each party will portray the results of its voter registration efforts for the general election as favorable.

But the best gauge of their success in that regard won't be available until a year from now, in advance of the 2009 primary, when party registration once again will matter in terms of the races for which voters can cast ballots -- and when there won't be a presidential nomination at stake in either party to induce voters to switch their registration.