Hotline OnCall's Reid Wilson points this morning to a possible warning sign that Sen. Blanche Lincoln is in trouble: she's polling below 50% in her Democratic reelection primary against Lt. Gov. Bill Halter.

Lincoln leads Halter 44% to 31% in the latest Daily Kos/Research 2000 poll for Arkansas, and while that's a comfortable lead, it could show Lincoln's support as vulnerable. Halter's campaign hasn't gotten fully underway yet--he only announced his candidacy March 1--and as things heat up, Halter's numbers could increase.

This race depends a lot on Halter's progressive backers. Organized labor and other progressive groups have shown they will dedicate significant resources toward spending and fundraising on Halter's behalf (as long as his bid looks competitive, that is).

Though Halter is a liberal challenger to an established (albeit struggling) incumbent, an upset victory for Halter in the primary wouldn't necessarily hurt Democrats in the general election, according to this poll. Both Lincoln and Halter trail the GOP favorite, Rep. John Boozman, by about the same margin, Daily Kos/R2K finds: Lincoln by 7%, Halter by 8%.