By Carl Hulse -- the New York Times

Today's United States Senate primary in Pennsylvania will test the appeal of the conservative Republican view on social and financial issues versus the more moderate party orthodoxy. So who is going to win: the Club for Growth or the Republican Main Street Partnership?

Those two competing Republican advocacy groups are heavily invested in the Pennsylvania race, which features the incumbent, Sen. Arlen Specter, considered the more moderate, trying to hold off a spirited challenge by a conservative, Rep. Pat Toomey. Polls had Senator Specter holding a narrow lead.

The Club for Growth made Senator Specter a top target even though he was supported by President Bush and the national party.

The Republican Main Street Partnership, on the other hand, is a backer of the dwindling number of Congressional Republicans who consider themselves fiscally conservative but not as far right on social issues as some of their brethren.

In a closing radio advertisement in support of Senator Specter airing in the Philadelphia and York markets, the group describes the senator as a "loyal Republican" and says: "A vote against Specter is a vote against the Republican Party." The ad even refers to the Club for Growth, saying Representative Toomey is backed by a group that says it does not care about the Republican Party.

Republican Party officials got behind Mr. Specter, who sometimes irritates them with his maverick views, partly because they fear Mr. Toomey would have a harder time holding on to the Senate seat in November. On Monday, the Club for Growth seemed to acknowledge some of that concern, calling on Mr. Specter to endorse his opponent should Mr. Toomey win.

"After tomorrow's primary, the Republican Party will need to be unified," said Stephen Moore, president of Club for Growth, adding, "If Pat Toomey wins the primary, having Arlen Specter's support would go a long way to helping him win in November."