The Senate is likely to be in session all night after they return to session after a nearly six-hour break at 12:01 a.m. Friday morning.

Democrats said the unusual nighttime session is necessary to get around the Republicans’ refusal to agree to any shortcuts to passage of both a Defense Department spending bill and a health care reform measure.

One senior Senate Democratic aide said Majority Leader Harry Reid (D-Nev.) might also need to keep the Senate in session every night until Christmas in order to ensure passage of the health care bill.

Sen. Tom Coburn (R-Okla.), an ardent opponent of the Democrats’ health care measure, said Thursday that Republicans would do everything in their means to slow down or kill the measure.

The bill will “not solve the problems that we said we wanted to solve when we started out,— Coburn argued. “The vast majority of Americans are against this bill and rightly so. So we ought to do everything we can to make sure it doesn’t become law. And there’s nothing dishonorable about that.—

The partisan tension in the chamber has reached a near zenith, with neither party giving any procedural ground. A dust-up Wednesday over the GOP’s attempt to force the reading of a 767-page amendment, and the Democrats’ subsequent successful effort to shut that reading down, precipitated the conflict that threatens to keep the Senate in session almost continuously until Christmas and possibly beyond.

Both parties were guilty Thursday of hair-trigger objections to routine unanimous consent requests. For example, Sen. Mark Begich (D-Alaska) at one point objected to Sen. John Cornyn’s (R-Texas) request to speak for two more minutes to an empty chamber. Begich almost immediately withdrew his objection, however.

Meanwhile, the Senate held a rare vote Thursday evening to allow the chamber to recess until midnight, a procedural move needed for the chamber to hold a scheduled 1 a.m. cloture vote to end debate on the Defense spending bill. The 59-38 vote to recess broke down along party lines. Only 51 votes were needed to prevail. Typically, such requests are agreed to without a roll-call vote.

The vote to recess was delayed by about an hour, apparently because Reid was continuing to try to wrangle 60 votes for the health care bill. Immediately after the vote, Democrats filed into a Caucus meeting called by Reid, though the purpose of the meeting was unknown.

Reid filed a motion to end debate, or invoke cloture, on the Defense bill Wednesday in order to get around GOP objections to moving more quickly. Under the rules, one full legislative day, plus one hour, must pass before the Senate can vote on cloture. As a result, the chamber will need to recess for some period of time in order for a “new— legislative day to begin.

Though Reid hopes to file several cloture motions to end a GOP-led filibuster of the health care bill as soon as Saturday, Democratic aides said it was still unclear when they would be able to unveil an official cost estimate from the Congressional Budget Office. But that score could still come sometime Thursday evening. The CBO score has become crucial to securing the votes of all 60 Members of the Democratic Conference.

Democrats are likely to file three motions to end the Republican filibuster of the health care measure after final passage of the Defense bill on Saturday, but the rules would prevent them from voting on the first cloture motion until Monday.

In order to pass the health care package, Democrats believe they will have to file cloture on a final, massive package of Democratic amendments, a substitute amendment to the bill, and on the bill itself. If the majority succeeds in getting enough votes on the first cloture vote on the manager’s package of amendments, they will have to wait 30 hours — essentially until Tuesday — before formally adopting it. They would then likely proceed to a cloture vote on the substitute amendment and wait until Wednesday before adopting that proposal. Finally, a cloture vote on the bill itself would occur Wednesday with a final vote perhaps on Thursday, Christmas Eve.

Republicans, such as Coburn, have threatened to disrupt that timeline by forcing a full reading of what is likely to be a nearly 2,500-page bill or using other procedural tools at their disposal. If that happens, Democrats have indicated they might have to push some votes to the week between Christmas and New Year’s Eve.