LINCOLN DRAWS PRIMARY CHALLENGER IN ARKANSAS…. By most measures, Sen. Blanche Lincoln (D-Ark.) is the Senate’s most endangered incumbent. As Arkansas has moved sharply to the right, Lincoln, a center-right Dem, has struggled to keep her footing.

Her challenge is a familiar one for Democrats in deep “red” states. Lincoln has moved further to the right, but not far enough for conservative voters. And as she shifts away from Democratic Party commitments, she also loses the support of the party’s base.

This morning, Lincoln’s position got a little more complicated.

Arkansas Lt. Gov. Bill Halter says he is seeking the Democratic nomination to challenge U.S. Sen. Blanche Lincoln this fall.

Halter’s spokesman provided a statement Monday in which Halter said he would file papers for the Senate this week. Halter is the only Democrat to formally announce a challenge to Lincoln as she seeks a third term.

This isn’t entirely unexpected; Halter first started talking about the possibility of a Senate race over four months ago.

Lincoln isn’t the only incumbent senator to face a primary challenger this year, but she’s arguably the incumbent most susceptible to one. Polls show her struggling badly, and Democrats hoping to keep the seat may be looking for alternatives with a better chance.

Enter Bill Halter, a fairly popular lieutenant governor from the party’s populist wing, whose announcement message positions him as a candidate running against a “broken” Washington system, bogged down in “gridlock, bickering, and partisan games.”

I’d be surprised if Halter’s announcement prompted Lincoln to rethink her re-election plans. In fact, she’s likely to enjoy strong support from the party establishment, as incumbents seeking re-election nearly always do.

But keep an eye on progressive institutions like labor unions, who’ll likely give Halter a serious look. Also note that the some of the netroots have already begun bolstering the primary challenger, with Progressive Change Campaign Committee, Democracy for America,, and Daily Kos planning to raise $500,000 for Halter this week.

Also watch to see if Lincoln follows the example set by Specter and Bennet, and starts moving back to the left to shore up Democratic support.

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Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.