LITTLE ROCK, Ark. (AP) — Democratic U.S. Sen. Blanche Lincoln, a moderate already considered one of the Senate's most vulnerable members, is facing another roadblock in her bid for a third term: a challenge from the left side of her own party.

Arkansas Lt. Gov. Bill Halter, a Conway native, announced Monday that he's seeking the Democratic nomination for Lincoln's seat, becoming the first in the party to formally challenge Lincoln and underscoring a schism in an election year shaping up to be difficult for the party in power.

"Washington is broken. It's working for the special interests, not Arkansas families," Halter said in a statement.

Eight Republicans already have announced interest in the Senate seat as Lincoln's popularity wanes in the GOP-leaning state. She has been under has been under pressure in Washington to support President Barack Obama's health care overhaul.

Groups on the left also have criticized her stances on labor and air pollution regulations. The liberal group had urged Halter to challenge Lincoln in the May 18 primary.

"I know that I am the target of both political extremes, but that's what makes this campaign so important to all of us," Lincoln said in a statement. "This Senate seat belongs to Arkansas, not to outside groups that are angry I don't answer to them."

Lincoln is just the latest vulnerable Democratic Senate incumbent to draw a primary challenge. Sens. Arlen Specter in Pennsylvania and Michael Bennet in Colorado are facing primary opponents — despite White House efforts to clear the field for them. And Sen. Kirsten Gillibrand in New York may soon find herself in a primary fight against former Rep. Harold Ford Jr.

In those cases, declared and undeclared Democratic primary challengers are hoping to seize on anti-incumbent sentiment that's running strong across the electorate. In Washington, Democratic Party officials fear that primaries will weaken the eventual nominee ahead of the November general election, and they have sought to minimize the number of primary races, particularly in the Senate.

Republicans seized on Halter's announcement as an opportunity to call for a change in Washington.

"Unfortunately for the Democrats, Lincoln and Halter are both completely out of step with Arkansas' mainstream values, and voters in this state have overwhelmingly rejected President Obama's out-of-control spending agenda in Washington," Amber Marchand, a spokeswoman for the National Republican Senatorial Committee, said in a statement. "Arkansans are tired of the status quo under the Obama Administration and they are looking to elect a Senator who will fight for much-needed checks and balances in Washington this November."

Halter, a one-term lieutenant governor, is a former Clinton administration official, having served as a deputy commissioner and acting commissioner of the federal Social Security Administration. He was elected lieutenant governor in 2006 after briefly considering a run for governor against Mike Beebe, who won the post.

Halter last year helped arrange a one-day free medical clinic in Little Rock organized by the National Association of Free Clinics. The medical clinic had been promoted by MSNBC's Keith Olbermann as a not-so-subtle jab at Democrats to support their party's health care reform efforts.

Lincoln has voted for the Senate version of a health care reform bill but has said she is opposed to a government-run health insurance option as part of the overhaul.

She has opposed key union-organizing legislation and Obama's nominee for the National Labor Relations Board. Both positions have gained the ire of the Arkansas AFL-CIO, which backed her re-election bid six years ago.

Lincoln has worked hard to insulate herself from the souring mood across the electorate as voters become impatient with the lack of jobs, the soaring federal deficit and elected officials who seemed locked in partisan infighting.

She's been virtually running against her own party's agenda on controversial issues such as the health care overhaul and ambitious spending proposals. For instance, she has said she opposes using a special budget-related procedure to go around GOP health care overhaul opposition in the Senate — which is now emerging as the Democratic strategy for getting a final bill.

Her ability to withstand the crossfire is considered a sign of the Democratic Party's ability to maintain a foothold in GOP-leaning states in November.

Lincoln's vulnerability raises hopes for Republicans in a state where Democrats hold three of the four congressional seats, all the statewide offices and a solid majority in the state Legislature. That hope has been heightened by two of the state's Democratic congressmen — Vic Snyder and Marion Berry — scrapping their re-election bids.

The Senate landscape has shifted since the year began, with several states now considered competitive because of unexpected Democratic retirements.

Republicans would need to hang on to all of their seats — which is far from a certainty — and pick up 10 Democratic-held seats to take control of the Senate. Aside from Lincoln in Arkansas, the GOP also is going after Democratic incumbents in Nevada, Pennsylvania, Colorado. Republicans would add New York to that vulnerable Democrat list if they could just find a candidate to challenge Gillibrand. The GOP also is making strong plays for open Democratic-held seats in Illinois, Delaware, Connecticut, Indiana, and North Dakota.


AP National Political Writer Liz Sidoti in Washington contributed to this report.

Copyright 2010 The Associated Press.