Veteran GOP senator Thad Cochran of Mississippi, who is battling a conservative primary challenger, has a new political headache: a scathing appraisal and conservative rating from the American Conservative Union, one of the right's most influential assessors of Republicans.

In its new annual scorecard, which was shared with The Washington Post on Wednesday, the ACU shows Cochran, at 63 percent, with one of the group's worst ratings during the Obama presidency. That's more than 25 points lower than the ratings for many of Cochran's GOP colleagues during the same five-year period, including others facing primary trouble, such as Sen. Pat Roberts (R-Kan.), who has an 89 percent rating, and Senate Minority Leader Mitch McConnell (Ky.), who has a 94 percent rating.

According to Dan Schneider, the ACU's executive director, Cochran's rating suffered mostly due to his opposition to legislation reducing the size of government from 2009 to present, which is the timeframe the organization has chosen to focus on, as way of more broadly measuring a legislator's conservatism.

"We've combined very visible votes with less visible votes to get these numbers," Schneider said. "I think Thad Cochran is in big trouble with his Obama-era rating. It's just poor and on this grander scale, he gets a D-minus. Over the last five years, he has shown a penchant for big government."

The ACU scorecard, which will be released Thursday, is one of several such measuring sticks within the conservative movement. Other groups, such as the Club for Growth and Heritage Action, also publish scorecards as ways to inform conservative voters about Congress.Cochran had the third worst score among Senate Republicans on the Club for Growth's scorecard last week. The club is supporting his primary opponent, state Sen. Chris McDaniel.

The ACU's scorecard, however, is notable for being released only once a year, unlike other groups, like Heritage Action, which actively scores votes as a pressure tactic on Republican legislators and frequently adjusts its congressional ratings.

Beyond the Obama-era evaluation, the ACU's new scorecard looks at a series of votes from 2013, from the farm bill to appropriations, as a way of charting where Republicans land on the conservative map. The lowest-rated Republicans last year were Rep. Michael Grimm (R-N.Y.) and Sen. Susan Collins (R-Me.). Grimm earned a 27 percent rating, Collins a 28 percent rating.

According to Larry Hart, the ACU's director of government relations, "conservatism was on a decline in 2013 in Congress," with fewer members moving to the right than he has seen in previous years.

"Only 16 members of the House earned a perfect ACU conservative rating of 100 percent, the lowest since 2006," Hart wrote in a memo. "The number of senators with a perfect conservative rating of 100 percent decline from eight to three, Tom Coburn (Okla.), Ted Cruz (Texas), and Mike Lee (Utah)." Top scorers in the House included Jim Jordan (R-Ohio), Rep. Steve Scalise (R-La.), and Trey Radel, a Florida Republican who recently resigned from Congress following an arrest and struggles with substance abuse.

Next week, the ACU will host the Conservative Political Action Conference, an annual gathering of conservative activists and Republican leaders, in Maryland. New Jersey Gov. Chris Christie, former Alaska governor Sarah Palin, and Rep. Paul Ryan of Wisconsin will be among the dozens of GOP leaders and conservative power brokers who will speak at the event.