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DENVER — Rep. Mike Coffman, R-Aurora, reversed course Tuesday and vowed to support a clean continuing resolution to fund the government, breaking ranks with an obstinate House GOP bloc that has insisted on changes to Obamacare in exchange for passing a CR to end the ongoing shutdown.

At least, in theory.

Coffman, one of the most vulnerable members of Congress, faces a tough reelection fight next year against Democrat Andrew Romanoff.

Last week, Coffman told FOX31 Denver he was adhering to the House GOP position and that he was comfortable with the position even if it eventually hurt his reelection chances.

But Tuesday morning, in an Op-Ed piece published in several community newspapers, Coffman reversed course, revealing yet another crack in the House GOP unity around the unrealistic strategy of holding the government hostage until Democrats agreed to the de-funding of the Affordable Care Act.

“I have done my best to delay, de-fund and dismantle all or parts of Obamacare because I believe that much of it will be harmful to this country in the long run,” Coffman wrote. “However, the debate over attaching Obamacare to a spending bill must end and I will argue before my colleagues in the House that we need to pass a “clean” spending bill to immediately reopen the government.”

Coffman also blasted “extremists” within both parties and took aim at the conservative lawmakers who pushed the strategy of forcing a shutdown.

“It concerns me that some Democrats appear to be ‘rooting for a shutdown’ because they think it helps them politically,” Coffman wrote. “And likewise, it also concerns me that some Republicans used the flawed strategy of a shutdown for what appears to be nothing more than an exercise in national fundraising.”

Coffman’s reversal adds to the pressure on House Speaker John Boehner to bring a clean CR to the floor.

On Monday, President Obama and Democratic Senate Majority Leader Harry Reid both dared Boehner to bring the CR up for a vote, knowing that it was likely to pass with mostly Democratic votes.

Boehner, should he give in, could face an insurrection within his own caucus, where a majority of members, most of them from relatively safe GOP districts, are pushing him to stay the course, even with the House GOP’s national disapproval rating spiking Monday to 70 percent.

With 200 Democrats already committed to voting yes, Coffman now makes 18 Republicans willing to break ranks — that puts the vote count at 223 yes votes, with 217 needed to pass.

But few are willing to truly break ranks with leadership, at least so far.

Romanoff: ‘Actions louder than words’

When the House voted Tuesday afternoon on another partial spending bill — this one would have funded the National Parks Service — Coffman opted not to support a Democratic effort to replace that legislation with the clean C.R. he just said he’d support.

Had he and the other House Republicans who say they support the clean C.R. voted with Democrats on this appeal, it could have allowed a vote on the clean C.R. by Tuesday evening.

“Actions speak louder than words,” Andrew Romanoff, Coffman’s Democratic challenger, told FOX31 Denver Monday. “Editorials won’t end the shutdown. A discharge petition will. Sign the petition. Take a stand.

“In the House I led, we disagreed with the governor — often. But we didn’t shut down the government. We found common ground. Congress could learn a lot from Colorado.”