Tuesday Preview.

Tuesday will be the biggest primary election day of the year thus far. Arkansas, Pennyslvania and Kentucky all have Senate primary contests:

In Arkansas, Democratic Senator Blanche Lincoln will defend her seat against a leftist primary challenger, Bill Halter. Also in Arkansas, Republican Congressman John Boozman is set to finish off some light competition for his party’s Senate nomination. Lincoln's seat looks likes a likely turnover in favor of the Republicans in the fall, regardless of the results on Tuesday.

In Pennyslvania, Republican-turned-Democrat Senator Arlen Specter will defend his seat in a primary against Congressman Joe Sestak, who had planned to run for the seat before Specter’s party switch. Notably, Specter is the only Senator to have been endorsed by both President Bush and Obama (in different elections.) Former Congressman Pat Toomey is the lone Republican running for Specter's seat and he also had planned to challenge Specter (...in the Republican primary) before the party switch.

In Kentucky, doctor Rand Paul looks to finish off his upset of early establishment favorite Trey Grayson. Also in Kentucky, two Democrats are in a tight contest for the right to be in the Senate general election, which the Republicans seem certain to win.

But the most notable race on Tuesday is a "general-special-election" House race in Western Pennyslvania. There is a tight contest going to fill the seat vacated by the late John Murtha. Should the Democrats lose this deep-blue-union-country seat, it will be a sign of things to come this fall. If the Democrats can not hold Pennyslvania’s 12th District, very few places will be safe for them in November.