Senate Republicans may be helping Sen. Mark Udall's re-election by signing an anti-earmarks letter he's circulating.

Despite being a top target this fall, the Colorado Democrat hasn't had much trouble finding Republicans to co-sign a letter he wrote with GOP Sen. Tom Coburn of Oklahoma that calls for continuing a moratorium on congressional earmarks.

According to the latest tally on Coburn's website, 23 Republicans in the Senate and seven in the House have signed the letter. Sen. Claire McCaskill of Missouri and Rep. Jim Cooper of Tennessee were the only Democrats to sign on.

Most Republicans are publicly opposed to budget earmarks, so it's not unusual that the Coburn-Udall letter would attract their support. But some on the Hill are surprised that so many have signed a letter that Udall could use to boost his prospects in the midterm elections.

His expected opponent, Republican Rep. Cory Gardner, has made Udall's dependable liberal voting record a line of attack. Recent public polls show the two locked in a tight race, and Udall could be in trouble if Colorado's purple, independent-minded electorate decides that he's been too reflexive in his support for Senate Majority Leader Harry Reid, D-Nev., and President Obama's agenda.

For Udall, advertising his collaboration with Coburn and more than two dozen other Republicans on an issue that animates conservatives and is generally opposed by his own party in Washington could prove an effective counterargument. The earmark moratorium has largely been pushed by Republicans, although voters of all stripes have criticized this method of budgeting, which allows individual lawmakers to direct money to favored projects.

“More than three years ago, members of Congress in both chambers from both parties stood together to impose a moratorium on the practice of congressional earmarking,” read a May 1 press release from Coburn and Udall announcing the circulation of the letter. “The American people celebrated this bipartisan leadership.”

Though it's clearly a sign of the bipartisanship many voters claim to crave, some Republicans on Capitol Hill are murmuring that working with Udall in such a public way is not exactly smart political strategy if their goal is help Gardner oust the Democrat in an election that is six months away. Republicans need to flip six seats to win the Senate majority, and the Colorado race could be pivotal.

In an email exchange, the Gardner campaign declined to directly address the matter, saying only that Udall was trying to “transform” is record on earmarks. A spokesman for Coburn said the Oklahoman is willing to work with anyone of any party that shares his goals.

“Dr. Coburn has always been more interested in the long-term health of the country than short-term political considerations. He’s happy to work with anyone who wants to maintain this ban,” Coburn spokesman John Hart said.