WASHINGTON — Today may be a holiday for federal government workers, but not for federal government watchdogs with Nevada Republican Sen. John Ensign in their sights.

A handful of groups dashed off a letter this morning to Senate Majority Leader Harry Reid, urging him put a stop to Ensign’s legislative maneuvering that has blocked a bill requiring senators to join the 21st century by filing their campaign reports electronically.

Ensign outraged the good government types earlier this year when he single-handedly halted the bill that has gained widespread support from Republican and Democratic senators.

Senators operate under an arcane rule that lets them file their campaign reports by snail mail, leaving the public in dark about how much money senators are collecting and spending.

The loophole is especially troubling in the final days of a campaign when voters have no way of knowing until after Election Day who is contributing to the candidates — be it big tobacco or anti-war activists.

Ensign says he supports the electronic filing bill, and just wanted to attach an amendment that would require groups that file ethics complaints against senators to disclose their big financial backers. Anonymous complaints can be done for political reasons, he has said, and pose a growing problem.

But the campaign finance reformers see Ensign’s amendment as nothing but a stall tactic, a poison pill to derail the bill they have been trying to pass for four years. Ensign is the head of the committee in charge of electing Republicans to the Senate in 2008, and could benefit if campaign donations are kept secret, they say. They further believe Ensign’s amendment is being orchestrated by Republican Senate Minority Leader Mitch McConnell of Kentucky, though Ensign has said he and the leader have worked together.

The letter sent this morning comes from a politically diverse group of non-profit organizations, from the Alliance for Justice to the American Conservative Union. They say Ensign’s amendment is retaliatory in nature, and would put a chill on the groups’ ability to file ethics complaints.

They believe Ensign’s amendment would fail if brought to the floor, and are asking Reid and McConnell to call the vote — or simply drop the amendment. “In view of this broad condemnation of the Ensign Amendment from major nonprofits spanning the ideological spectrum, Senator Ensign should agree to drop his obstructionist amendment,” said a statement released by Michael Malbin, executive director of the Campaign Finance Institute, which has been fighting for electronic disclosure. “If he wants to pursue the issue despite the broad opposition, let it stand on its own.” Ensign has said previously he believes his amendment would pass, and has asked for floor time.