When Sen. Tom Coburn (R-Okla.) served notice Tuesday that he would stymie further consideration of spending bills until appropriators agree to new transparency rules, it marked a return to the combative style the Oklahoman has largely avoided this year.

Pointing to appropriations conferees’ decision to withhold from the public a host of reports required under the Energy and water spending bill, Coburn warned his colleagues that he would throw a roadblock before any conference reports coming to the floor.

“I serve notice on the Senate that any conference report that does not have transparency, which I will offer and have offered to every bill that comes back from a conference, I will do everything I can to block it. … I will not give up until such time as the American people truly get to see a transparent government,— Coburn said on the floor.

Coburn acknowledged that he has used filibusters less than in the past, but said he is particularly upset that appropriators stripped the transparency language out of the Energy and water bill — even though they left it intact in the Defense spending bill.

“There’s no reason for not having that transparency and that’s why we’re going to slow things down. … It was in the Defense bill and it stayed,— Coburn said.

Despite a reputation as a master of the chamber’s floor rules and a penchant for using them to tie Democrats in procedural knots, Coburn hasn’t been a major distraction for Democrats this year. That represented a dramatic shift from a year ago when Coburn and Senate Majority Leader Harry Reid (D-Nev.) engaged in a public — and often personal — war on the Senate floor.

After coming to the Senate in 2004, Coburn quickly made a name for himself as one of the chamber’s leading conservative firebrands. Coburn and his top aides pored over the Senate’s arcane rules and used them to create legislative blockades against earmarks and other spending that Coburn deemed unnecessary.

Coburn’s efforts often met with bipartisan consternation — and he often ran afoul of both Reid and Minority Leader Mitch McConnell (R-Ky.).

But since the beginning of the year, Coburn has abandoned the use of holds and filibusters, largely because of Reid’s decision to allow votes on GOP amendments.

“I think part of it is we’re amending things … for better or for worse, we’ve had votes on a number of amendments,— a senior GOP leadership aide said, chalking up the change to a shift in how Reid manages the floor.

“The reason it’s not been as hot of a topic as it was last year is the process the Majority Leader has chosen to go forward with,— the GOP leadership aide said. While Republican amendments have rarely succeeded, Republicans like Coburn have decided to respect the legislative process rather than block bills.

A source close to Coburn agreed, explaining the lawmaker and his staff are “very responsible … they know to be an effective last line of defense for taxpayers, you can’t fight every little battle.—

This source, however, warned that Coburn would look to return to his more combative ways whenever he feels slighted, as he does in the case of the transparency legislation. “If they feel like they’ve been played,— the source said, Coburn’s office will actively work against passage of legislation.

National Republican Senatorial Committee Chairman John Cornyn (Texas) acknowledged that Republicans have not tried to filibuster as many bills or slow-walk legislation because they have gotten more votes this year. However, he argued that Reid’s decision has not been driven by some sense of equality — but by the simple fact that Republicans no longer have the numbers to actually filibuster bills.

“Democrats are happy to allow votes when they know all these votes will go their way,— Cornyn said.