Election season begins afresh: Four states are holding their primaries on Tuesday, plus there’s a special election for the House seat held by late Rep. John Murtha. It’s figuring to be a bad day for incumbents and party favorites. In Pennsylvania, left-wing challenger Joe Sestak has a good shot of unseating incumbent Democratic Senator Arlen Specter (Nate Silver puts the odds of a Sestak victory at 3:1); in Arkansas, left-wing challenger Lieutenant Governor Bill Halter can force a runoff by keeping incumbent Democratic Senator Blanche Lincoln below 50 percent of the vote; and in Kentucky, Republican Rand Paul seems likely to upset Senate Minority Leader Mitch McConnell’s handpicked Senate candidate, Trey Grayson. In the special election for Murtha’s old House seat, Tom Schaller at FiveThirtyEight.com predicts a “nail biter” and writes, “as one of those 83 McCain-Democrat districts, PA-12 is precisely the sort of district Republicans have in their sights for 2010... and precisely the kind Democrats need to prove they can hold onto if they want to keep their majority or even maintain a workable majority in the House next year.”