Arlen Specter switched from R to D in Pennsylvania and is being challenged in the Democratic primary, apparently, by Congressman Joe Sestak (D-PA-7). 

Pat Toomey is the considered by many as the leading Republican candidate.  Peg Luksik is another Republican candidate.

There are fault lines emerging on the trade issue debate on the Republican side.  Toomey chaired the Club for Growth until he announced his Senate candidacy.  The Club for Growth has vociferously supported current free trade policies and all the currently proposed trade agreements.

Luksik has announced support for a smarter, reformed trade policy by signing on to the Fixing America's Economy plan (which was endorsed by CPA).  This is a good sign that the trade and economic debate will be highlighted during the campaign on the Republican side.  The primaries in Pennsylvania are May 18, 2009.

Luksik's campaign website is here.

(CPA does not endorse or oppose any political candidates).