The biggest day on the primary calendar all year takes place tomorrow, with elections in four states and some of the most interesting storylines of the 2010 campaign. Activists on the left and right have battles against the establishment which will be decided tomorrow, and Democrats and Republicans have the opportunity to declare victories which they will spin into a narrative about November. Here’s a guide to the election:

• Pennsylvania: The biggest story here is the Democratic primary for Senate. For months, Joe Sestak couldn’t gain any traction against party-switcher Arlen Specter, but suddenly in the last month, he surged in the polls, and now the race is seen as a dead heat, with enough undecideds to swing the race in one direction or the other. The question is whether Specter’s establishment support, with the DNC’s campaign organization and President Obama himself working on his behalf, will juice turnout enough to drag him over the finish line, or whether Sestak will get the bulk of the undecideds in a lower-turnout race and ride his current momentum. Specter’s experience argument, that his 30 years in the Senate represents a better deal for Pennsylvanians, may resonate as well. Chris Bowers predicts a Sestak victory:

None of those leads make Sestak anywhere close to a sure thing. However, Specter is only competitive in this primary due to his ongoing advantage in name ID, an advantage which averages 20% across five of these six polls (excluding F & M). That means Specter still has a big advantage among low information voters (who have still never heard of Sestak), and as such will need high Democratic turnout to win (since these low-information voters are less likely to vote). With Democratic turnout sharply down in primaries do far this year, that ain’t too bloody likely.

I expect Sestak to win as this point, and to become the first Democrat to defeat an incumbent backed by Obama in a primary.

Specter’s erratic behavior in the final days, including trying different messages on different regions of the state and claiming that his party switch was a matter of principle because he could easily have won a Republican primary (he admitted the switch after READING POLLS SHOWING HIM LOSING such a primary), have led insiders to despair about his chances. Bob Schieffer told local Philly news that the White House is bracing for a Specter defeat. But I think it’s a little quick to anoint Sestak the winner; this is a very tight race.

There are contested primaries on both sides for Governor as well, with Republican Attorney General Tom Corbett and Democratic Allegheny County Executive Dan Onorato the favorites.

• PA-12: The special election to replace John Murtha coincides with the statewide primary, and because of the Sestak-Specter race this has the possibility of drawing more Democrats to the polls. However, right now the race is a pure toss-up between Murtha staffer Mark Critz and businessman Tim Burns. Both party committees have poured money into the race, but the circumstances around it slightly favor Critz. In addition to the heavier attention on the Democratic primary, Burns has a tough primary of his own, against 2008 nominee Bill Russell, while Critz faces token opposition. It’s entirely possible that Burns wins the special election and loses the primay, making him an instant lame duck.

• Arkansas: Bill Halter has been chasing Blanche Lincoln ever since he announced his campaign just a couple months ago. Halter received strong support from the netroots and the labor movement. The most likely scenario at this point looks to be a runoff; with right-wing Democrat DC Morrison also in the race (and getting residual support from the negative primary tactics of both the other candidates), it’s possible that neither Lincoln nor Halter will get the 50% + 1 needed to avoid a second round under Arkansas law. That runoff would take place on June 8. Lincoln positions herself as in the sensible center, disliked by the left and right, but in a potential second-round matchup it would be just her and Halter, and her fealty to wealthy and corporate interests would be on full display in an election likely to feature low turnout.

Because of Rep. John Boozman running for the Senate seat on the Republican side (in a crowded field he also may not be able to avoid a runoff), and retirements from Reps. Vic Snyder and Marion Berry, three of the four House seats in the state are open, with many candidates seeking the seat in primaries.

• Kentucky: Both the Republican and Democratic Senate primaries have interesting storylines. Rand Paul has tracked the anti-establishment wave in the GOP and is expected to win big over Secretary of State Trey Grayson, who was endorsed by Mitch McConnell and most of the bigwigs in the Kentucky Republican Party. Among Democrats, ConservaDem Lt. Governor Dan Mongiardo has led the more mainstream liberal Attorney General Jack Conway through most of the year, but Conway has closed well and now it’s a dead heat. A Conway-Paul matchup would offer a real alternative in Kentucky, and polls have shown that race to be competitive.

• Oregon: There’s not quite as much drama in Oregon, where Ron Wyden is expected to cruise to re-election. The Governor’s race features former chief executive John Kitzhaber expected to win the Democratic primary, and former NBA journeyman and terrible free throw shooter Chris Dudley expected to win on the Republican side.

Beyond the marquee races, there are plenty of other primary scenarios. Steve Singiser has you covered on those. He also mentions that the Hawaii special election for the 1st Congressional district, where Republican Charles Djou could benefit from a split Democratic vote in the unusual first-past-the-post, no-primary election and pick up a blue seat, will be completed on Saturday.