Sen. Patrick J. Toomey, R-Pa., the former president of the conservative Club for Growth , is seeking re-election in a state that hasn't picked a GOP nominee for president since 1988.
So will Toomey — who has been endorsed for re-election by this outside group — moderate his votes or platform to appeal to the political center of the Keystone State ahead of 2016? It's one of the most interesting questions of the 2016 cycle — and it's just one reason why the Pennsylvania Senate race tops Roll Call's list of the Top 10 Races to Watch in 2016.
Over the next two weeks, Roll Call will reveal daily a key race for this nascent cycle and examine its dynamics. Here's another reason the Keystone State tops the Senate race landscape: Will anyone challenge former Rep. Joe Sestak, D-Pa., for the nomination?
In 2010, Toomey's threat to wage a primary challenge to the late-Sen. Arlen Specter, then a moderate Republican, was enough to make the longtime senator switch parties to become a Democrat. Specter eventually lost the Democratic primary to Sestak.
Sestak later went on to lose to Toomey by a 2-point margin in a dismal year for Democrats.
No matter whether Toomey faces Sestak or another Democratic nominee, he will find himself with a challenging path to re-election.
If Democrats run strong in the presidential race, Toomey's challenge to attract split-ticket voters only increases. That tough task makes him one of the most vulnerable senators in the country, and a top target for Democrats as the party looks to net five seats to ensure Senate control.
Editor's Note: Over the next two weeks, Roll Call will unveil its Top 10 Races to Watch for 2016. The final edition will run Jan. 2 with the full list. Related Stories: House Democratic Losses Deplete Bench of 2016 Senate Recruits Club for Growth President Steps Down Pennsylvania Democrats Hesitate on Sestak Senate Bid Club for Growth Endorses 6 Senators for 2016 The 114th: CQ Roll Call's Guide to the New Congress Get breaking news alerts and more from Roll Call in your inbox or on your iPhone.