Nearly every senator returned to Washington Monday evening in time to attend a closed-door joint caucus to discuss filibuster rules.

Ninety-seven senators participated in the mandatory quorum call Senate Majority Leader Harry ReidHarry ReidIf Gorsuch pick leads to 'crisis,' Dems should look in mirror first Senate confirms Mulvaney to be Trump’s budget chief Democrats declare victory after Puzder bows out MORE (D-Nev.) requested ahead of the 6 p.m. caucus in the Old Senate Chamber.

The GOP cloak room said Sen. Marco RubioMarco RubioRepublicans play clean up on Trump's foreign policy Top Dem: GOP is terrified of Trump McConnell on Trump: 'I'm not a fan of the daily tweets' MORE (Fla.) was the only missing Republican, while Sens. Jeanne Shaheen Jeanne ShaheenMattis on rise in Trump administration Scott Brown being considered for ambassador to New Zealand: report Senate advances Trump's Commerce pick MORE (N.H.) and Bob MenendezRobert MenendezSteve Mnuchin, foreclosure king, now runs your US Treasury Senate Dems move to nix Trump's deportation order Senators to Trump: We support additional Iran sanctions MORE (N.J.) were the absent Democrats.

Reid has threatened to use the “nuclear option” — changing the Senate rules to allow presidential nominations to proceed with a simple-majority vote instead of a 60-vote super-majority — in order to thwart Republican obstruction on seven of President Obama’s cabinet nominees. 

Sen. John McCainJohn McCainTrump’s feud with the press in the spotlight Republicans play clean up on Trump's foreign policy Graham: Free press and independent judiciary are worth fighting for MORE (R-Ariz.) told reporters heading into the joint caucus that lawmakers had agreed to the outlines of a compromise on nominees although a senior Democratic aide disputed the claim.

“We’ll go over those outlines,” said McCain. “I think it’s very important that we give those proposals that our supported by a number of Republicans at least the consideration of our colleagues on both sides of the aisle.

McCain said some Democrats are familiar with the details of the proposed compromise.

“They know it and many of them are supportive of it but we’ll see,” he said.

A senior Democratic aide disagreed strongly.

“There’s no deal,” said the aide, who explained that McCain proposed swapping in two nominees to the National Labor Relations Board to replace Sharon Block and Richard Griffin, Jr., whom President Obama put on the board with recess-appointments. The D.C. Circuit Court later ruled the move unconstitutional.

McCain has shared the plan with senior White House officials.

“They’d like to see it resolved as well but they know it’s largely a Senate issue,” he said.

In the fight over Obama's nominees, Democrats have argued that the president should be able to select his own team and that executive branch nominees shouldn’t be held to a 60-vote threshold. Reid has emphasized that the rules change would not cover judicial nominees. 

Republicans have argued that they are simply exercising the rights in the Constitution for the Senate to “advise and consent” on executive nominees and that changing the rules would harm minority rights in the Senate.

They've also threatened to retaliate if they regain the Senate majority.

Senate Minority Leader Mitch McConnellMitch McConnellRepublicans play clean up on Trump's foreign policy Americans brimming with optimism on the economy McCain hopes Americans can be confident GOP-controlled Congress can investigate president MORE (R-Ky.) said last week that he was disappointed the joint caucus was scheduled for Monday instead of Tuesday because not all lawmakers return from spending the weekends in their homes states by Monday evening.

Seven nominees are at issue: Richard Cordray as director of the Bureau of Consumer Financial Protection; Griffin, Block and Mark Pearce to the National Labor Relations Board; Fred Hochberg to be president of the Export-Import Bank; Thomas PerezThomas E. PerezClinton’s top five vice presidential picks Government social programs: Triumph of hope over evidence Labor’s 'wasteful spending and mismanagement” at Workers’ Comp MORE to be secretary of Labor; and Gina McCarthyGina McCarthyObama EPA chief: Pruitt must uphold ‘law and science’ Overnight Energy: Congress does away with Obama coal mining rule GOP suspends rules to push through EPA pick despite Dem boycott MORE to be administrator of the Environmental Protection Agency.

Cordray and the NLRB nominees are the most controversial.

Procedural votes on those nominations are expected to start Tuesday, but senators are hoping to reach a deal to allow up-or-down votes to avoid the “nuclear option.”

This story was updated at 6:39 p.m.