Open season?

Voters in New York, New Jersey, Virginia and Massachusetts have fired warning shots at the political establishment over the past seven months. Today's primary elections in Arkansas, Pennsylvania and Kentucky could make it open season on incumbents -- and define the course of the 2010 campaign.

By tonight, officeholders and insiders everywhere will know whether they'll have a fighting chance to keep their jobs, or whether they'll have nowhere to hide.

By tonight, two sitting U.S. senators could be sent packing into retirement.

In Arkansas, Democratic Sen. Blanche Lincoln is fighting a primary challenge against Lt. Gov. Bill Halter and businessman D.C. Morrison. Sen. Lincoln, supported by President Obama, might well finish first today, but if she fails to get 50 percent of the primary vote, she'll have to win a June 8 runoff against the second-place finisher for the right to be the underdog against the GOP nominee in November.

In Pennsylvania, Democratic Sen. Arlen Specter -- a career Republican who switched parties because he faced certain defeat in his state's GOP primary -- is in a dead heat against Rep. Joe Sestak. Sen. Specter has campaigned with his state's establishment -- sitting politicians and unions -- and lost a lead of more than 20 percentage points as a result.

Kentucky's primary of note is the Republican Senate race to replace the retiring Jim Bunning. Secretary of State Trey Grayson has the backing of Senate Minority Leader Mitch McConnell, R-Ky., the party elders and the Chamber of Commerce. But outsider Rand Paul, the son of Texas Rep. and former presidential candidate Ron Paul, is leading thanks to the support of the Tea Party movement.

The stakes in these three races are incredibly high. The amount of money being poured into those states, especially by unions and business groups, says as much. Sen. Lincoln's primary race will top $10 million in expenditures, making it one of Arkansas' most expensive elections ever. Just a few years ago, an incumbent with Sen. Lincoln's clout never would have faced a primary in the first place.

Will the endorsement of President Obama be a blessing or a burden? Will the Tea Party gain enough momentum to swing pending Senate primaries in Nevada, Arizona and elsewhere? Or will establishment candidates get the mixed results they clearly need to have a shot in November?

Incumbents will be paying close attention tonight -- from a safe distance, behind plenty of cover.