Blanche LincolnBlanche Lincoln Arkansas Lt. Gov. Bill Halter (D) is mounting a primary challenge against conservative Democrat Blanche Lincoln for the U.S. Senate seat she’s held for two terms.

Environmentalists and progressives have it in for Lincoln, angry over her opposition to high-profile Democratic issues like a public option in health care and cap-and-trade. The Sierra Club is running ads bashing her for trying to stall new fuel-economy rules on behalf of Big Oil. (She’s been the top recipient of campaign contributions from the oil and gas sector for the past five years.) MoveOn is sending emails calling Lincoln “one of the worst Democratic senators” and urging its members to donate to Halter.

Recently named to the League of Conservation Voters’ “Dirty Dozen,” Lincoln has taken heat for working to block the EPA from regulating greenhouse-gas emissions. She was also one of 10 Democrats who contributed to the downfall of the Lieberman-Warner Climate Security Act in 2008. Along with other swing-vote senators, Lincoln signed a letter [PDF] outlining the reasons she would not support the bill. (Kate Sheppard has more on Lincoln’s climate and energy record.)

Meanwhile, Halter seems to be getting somewhat of a free pass. MoveOn, unions, and other liberal groups have helped him amass a war chest of more than $5 million, but he’s managed to get this far with little mention of the environment. His campaign website is decidedly ambiguous on climate and energy, focusing instead on education and jobs.

Bill Halter. Bill HalterIn an interview with Talking Points Memo, Halter glosses over environmental issues. He gives a blunt “no” when asked if he would share Lincoln’s position on blocking the EPA from regulating greenhouse gas emissions, but remains vague about cap-and-trade, a point not lost on TPM:

On the issues, Halter often came down on the left-hand side of the line. He told me he likes candidates that talk specifics and in most cases he put his money where his mouth was, offering detailed answer on a number of policy fronts. One notable exception was cap-and-trade, where … he didn’t offer a specific policy stance, instead talking at length about developing alternative energy resources to better the environment and the economy. But he said “there are significant changes that need to be made” to the House cap-and-trade bill before he could support it. Halter ended our interview before he had a chance to elaborate on what those “significant changes” are.

Whoever wins the nomination will face a tough general-election race against the likely Republican nominee, U.S. Rep. John Boozman (R), who is currently polling well ahead of both Lincoln and Halter.