Americans for Tax Reform, a conservative anti-tax group, is taking shots at Sen. Tom Coburn (R-Okla.) for donating to Senate Democrat Joe Manchin of West Virginia.

Grover Norquist, president of the group, noted in a Twitter posting Sunday that Manchin had a 15-percent rating by the American Conservative Union in 2011.

That was followed by a salvo of tweets from John Kartch, director of communications at Americans for Tax Reform, taking Coburn to task.

{mosads}One post highlighted Coburn’s and Manchin’s shared support of the Bowles-Simpson deficit-reduction plan. Another cited House Budget Committee Chairman Paul Ryan’s (R-Wis.) estimate that Bowles-Simpson would raise taxes by $2 trillion over 10 years.

“Coburn says he gave money to Manchin (D) because Manchin supports Simpson-Bowles, a $1 – 3 TRILLION tax hike,” Kartch wrote in another tweet.

Coburn told The Hill he gave $250 to Manchin’s campaign on June 27 because his colleague shares his concerns for the long-term interests of the nation.

“I think he votes thinking about the long-term interests of the country. We don’t agree on everything but he’s a good guy,” Coburn said.

Coburn views the nation’s $16 trillion debt as a major threat and has even chided reporters for asking questions about presidential campaign politics instead of focusing on fiscal policy.

Coburn was one of three Senate Republicans to vote for the plan proposed by the National Commission for Fiscal Responsibility, co-chaired by former White House chief of staff Erskine Bowles and former Sen. Alan Simpson (R-Wyo.) in December of 2010.

Manchin has emerged as another strong supporter of the Bowles-Simpson plan.

“I’ve always been for the Bowles-Simpson template,” he told reporters last week. “I’ve always been for that. I’m very hopeful and continue to remain hopeful.”

Coburn and Norquist clashed heatedly last year over an effort by the Oklahoma senator to eliminate a $6 billion tax break for the ethanol industry. Coburn has criticized the tax credit as a wasteful subsidy but Norquist argues tax deductions should not be eliminated unless offset by other tax cuts.

Americans for Tax Reform highlighted a recent quote Coburn gave to The Oklahoman predicting that Americans would support closing tax loopholes.

“Quiz: Who said: ‘They’re happy as loyal Americans to pay more taxes. And they will under any plan that I put forward.’ A. Obama or B. Coburn,” Kartch wrote.

Coburn believes the public will support the elimination of many loopholes if it results in a simpler tax code and reduces the deficit.

“They’re willing to give that up if they see some clarity and certainty in the future. What they don’t want to do — and I can’t blame them since the government’s twice the size it was 11 years ago — they don’t want the increased taxes to go for growing the government more,” Coburn told his home-state paper.

“They’re happy as loyal Americans to pay more taxes. And they will under any plan that I put forward,” he added.

The Bowles-Simpson plan calls for raising hundreds of billions of dollars in new tax revenue by eliminating niche tax breaks.

Americans for Tax Reform on Sunday tweeted a reminder that the Heritage Foundation called the plan “the highest sustained tax burden in American history”.

Relations between Norquist and Coburn have remained tense since last year’s blowup over ethanol.

Norquist said in an interview Friday that he has not made up with Coburn.

Americans for Tax Reform estimates the Bowles-Simpson plan would raise taxes by as much as $5 trillion over the next decade.

“That’s what he’s endorsed. I don’t think he knows it,” said Norquist.