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 Mason ReidTrump thanks Reid for warning Democrats not to underestimate him Reid warns Democrats not to underestimate Trump Harry Reid predicts Trump, unlike Clinton, won't become more popular because of impeachment MORE (D-Nev.) requested ahead of the 6 p.m. caucus in the Old Senate Chamber.

The GOP cloak room said Sen. Marco RubioMarco Antonio RubioHillicon Valley: GOP lawmakers offer election security measure | FTC Dem worries government is 'captured' by Big Tech | Lawmakers condemn Apple over Hong Kong censorship Lawmakers condemn Apple, Activision Blizzard over censorship of Hong Kong protesters Youth climate activists get Miami Beach to declare climate emergency MORE (Fla.) was the only missing Republican, while Sens. Jeanne ShaheenCynthia (Jeanne) Jeanne ShaheenGraham, Van Hollen introduce Turkey sanctions bill Senators fear Syria damage 'irreversible' after Esper, Milley briefing US envoy insists Syria pullout doesn't affect Iran strategy MORE (N.H.) and Bob MenendezRobert (Bob) MenendezPaul blocks Senate vote on House-passed Syria resolution House to vote on resolution condemning Trump's Syria pullback Rand Paul calls for probe of Democrats over Ukraine letter 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 Sidney McCainCummings to lie in state at the Capitol Elizabeth Warren should concern Donald Trump 'bigly' Lawmakers toast Greta Van Susteren's new show 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 McConnellAddison (Mitch) Mitchell McConnellMcConnell: Trump's troop pull back in Syria a 'grave strategic mistake' Overnight Defense — Presented by Boeing — Trump insists Turkey wants cease-fire | Fighting continues in Syrian town | Pentagon chief headed to Mideast | Mattis responds to criticism from Trump TSA head rules himself out for top DHS job   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 Edward 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 McCarthyRegina (Gina) McCarthyOvernight Energy: Dems subpoena Perry in impeachment inquiry | EPA to overhaul rules on lead contamination tests | Commerce staff wrote statement rebuking weather service for contradicting Trump Hundreds of former EPA officials call for House probe, say agency's focus on California is politicized It's time for Congress to address the 'forever chemical' crisis 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.