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 ReidDems search for winning playbook Dems face hard choice for State of the Union response The Memo: Immigration battle tests activists’ muscle 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 RubioOvernight Cybersecurity: Bipartisan bill aims to deter election interference | Russian hackers target Senate | House Intel panel subpoenas Bannon | DHS giving 'active defense' cyber tools to private sector Senators unveil bipartisan push to deter future election interference Puerto Rico's children need recovery funds MORE (Fla.) was the only missing Republican, while Sens. Jeanne ShaheenCynthia (Jeanne) Jeanne ShaheenSupreme Court to hear online sales tax case State official indicates US military role in Syria post-ISIS centered on Iran Overnight Health Care: Dems press HHS pick on drug prices | Alexander, Trump discuss ObamaCare fix | Senate Dems seek B to fight opioids | Maryland eyes ObamaCare mandate replacement MORE (N.H.) and Bob MenendezRobert (Bob) MenendezCongress must provide flexible funding for owners of repeatedly flooded properties Senate ethics panel resumes Menendez probe after judge declares mistrial Judge declares mistrial in Menendez bribery case 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 McCainMcCain rips Trump for attacks on press NSA spying program overcomes key Senate hurdle Meghan McCain says her father regrets opposition to MLK Day 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 McConnellSessions: 'We should be like Canada' in how we take in immigrants NSA spying program overcomes key Senate hurdle Overnight Finance: Lawmakers see shutdown odds rising | Trump calls for looser rules for bank loans | Consumer bureau moves to revise payday lending rule | Trump warns China on trade deficit 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 McCarthyGina McCarthyThe media’s tactics to silence science at Trump’s EPA Overnight Energy: EPA releases ozone findings | Lawmakers come out against Perry grid plan | Kids sue Trump on climate change Congress must come to terms on climate change regulation 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.