The big focus in Washington, D.C. tonight will be primary election results in Pennsylvania, Arkansas and Kentucky, as political experts get to see whether any more incumbents will go down to defeat.

So far, one Democrat has lost a primary in the House, Rep. Alan Mollohan (D-WV).  One Republican failed to even get in a primary in the Senate, Sen. Robert Bennett (R-UT).

Today, the most endangered incumbent is certainly Sen. Arlen Specter (D-PA).  

I just had a chuckle while writing that last sentence, because I instinctively typed (R-PA), reflecting the fact that Specter was a Republican for 45 years.

His opponent, Rep. Joe Sestak (D-PA), has been running ads hitting Specter on his party switch.  It has a sound bite and video where President Bush says nice things about Specter to cheers from the crowd, "Arlen Specter is the right man for the U.S. Senate."

The ad wraps up with the narrator slamming Specter one more time: "Arlen Specter switched parties to save one job: His.  Not yours."

We'll see tonight if Specter can survive this battle.  

Ironically, his main hopes lie with the Democratic machine from the city of Philadelphia and support courtesy of Gov. Ed Rendell (D), who has said Specter needs a good turnout from his supporters.

As we detailed yesterday, the Senate races in Arkansas and Kentucky will also be a focus tonight.

In Arkansas, Sen. Blanche Lincoln (D-AR) will try to avoid a runoff against the more liberal Lt. Gov Bill Halter.

In Kentucky, the news may be a defeat for the GOP Establishment, as Rand Paul (son of Rep. Ron Paul) tries to knock off Secretary of State Trey Grayson in the Republican Senate Primary.

Remember how rare it is that Senators lose in a primary.  The last one was in 2006, when Sen. Joe Lieberman (I-CT) went down - but remember - he came back to win again as an Independent in the November election.

If you're interested in monitoring the results, please join me tonight on Twitter - you can find me @jamiedupree