In a year when political protest has been defined by the Tea Party and challenges from the political right, tomorrow's Democratic run-off between Sen. Blanche Lincoln and Lt. Gov. Bill Halter has become 2010's marquee battle for progressives.

And progressives -- mostly in the form of labor, but also MoveOn and other outfits -- is investing heavily in the outcome. Labor alone has spent $6.76 million for Halter, and reports knocking on170,000 doors, made 700,000 phone calls, sent 2.7 million pieces of mail.

Given that Arkansas only has some 47,000 union members itself -- 4.2% of workers, second-lowest unionization rate in the country -- Lincoln has seized on labor's support as evidence of "outsiders" (read: carpetbaggers?) hijacking the race.

It's a disingenuous line of attack: Sen. Lincoln is the top recipient of PAC money of any politician in the country this election cycle. And most of this PAC money isn't from mom-and-pop operations in Arkansas; as Think Progress shows, it's a list of the big corporations who pushed Lincoln to embrace positions on health care, energy policy and labor law that inspired a challenge from the left in the first place:
 

$10,000 from Chevron
$5000 from ExxonMobil
$5000 from Occidental Petroleum
$4000 from British Petroleum
$7000 from Anadarko
$6000 from Bayer
$5500 from Blue Cross Blue Shield
$5000 from Boeing
$5000 from the U.S. Chamber of Commerce
$6000 from Comcast
$6500 from Goldman Sachs
$6500 from JP Morgan Chase
$7000 from Lockheed Martin
$8000 from Wal-Mart

Sen. Lincoln also ranks among the top 10 incumbent senators for having the most contributions coming from out of state.

Lincoln also has the support of the Democratic establishment: The party maxed out on its contributions to Lincoln's 2010 campaign, and the Big Dog himself, former President Clinton, is now criss-crossing the state for her.

So what will come of this defining showdown between a symbol of the centrist Democratic establishment and labor's all-in gamble for change?

The polls are too close to call. In a tight race, small election problems like the surprise decision in Garland County -- a Halter stronghold -- to eliminate 34 of 36 voting locations to "save money" could be a factor.

As for those who bet on political races, they're smelling an upset: As of this writing, the Intrade political prediction markets are only giving Lincoln a 17% chance of winning tomorrow.