Sequence of events:

1) Arlen Specter votes in favor of President Obama’s stimulus package.

2) Pat Toomey announces his challenge to Specter, backed by Club for Growth and other outside groups.

3) Fearing the primary challenge, Specter reaffiliates as a Democrat—just in time to cast the 60th vote in favor of the Affordable Care Act.

No primary challenge, no Obamacare. It’s that simple.