Washington (CNN) -- It's hard to imagine the atmosphere in Congress getting any more toxic, but it could if Senate Majority Leader Harry Reid carries out his threat to use the "nuclear option" to budge stalled presidential appointees forward.
A top Republican aide warns that important bills like the recently approved immigration measure would never pass in a post-nuclear Senate.
"If they blow the place up, then anything requiring a big bipartisan push like immigration will be impossible," the GOP aide said.
Republicans will protest by slamming the brakes on action in the Senate, current and former senior Senate aides from both parties predicted to CNN.
The poisoned atmosphere could stall passage of several important items moving through Congress like tax reform, judicial nominations, government spending bills and a debt ceiling increase. Even a relatively modest rewrite of student loan laws could be in jeopardy, meaning students headed to school this fall would have to pay higher interest rates.
Democrats control the Senate but don't have the 60 votes necessary to break a filibuster. Reid warned Republicans if they don't drop filibusters against Obama's Cabinet and agency picks, he will take the drastic step of changing Senate rules without Republican consent.
The change would prevent filibusters of executive branch nominations, allowing them to be confirmed on a simple majority vote.
A last-ditch effort to head off the crisis will come Monday night, when all senators are scheduled to attend a closed session that Republicans requested in the historic Old Senate chamber.
'Bipartisan cooperation will come to a grinding halt'
"A likely result of Reid triggering the nuclear option will be an erosion in the comity that makes the Senate the unique institution that it always has been," one senior Republican staffer told CNN. "It is very likely that bipartisan cooperation will come to a grinding halt."
A top Democratic aide said, "The immediate aftermath will be messy. The GOP will shut down the Senate and they will keep it locked down until there are tangible impacts from inaction. Only then, when people start to realize that their lawmakers are not working on the farm bill, immigration, budget and other issues, will they pick up the phone and call their lawmakers."
Jim Manley, a former top aide to Reid, said Republicans have a number of options to slow things down.
"I'm not so sure what else (Senate Republican leader) Sen. (Mitch) McConnell can do to slow down the Senate more than he already has, but I'm entirely confident he has any number of tricks up his sleeves," said Manley, who was with Reid in 2005 when Republicans, then in the majority, threatened to use the nuclear option to prevent filibusters against President George W. Bush's judicial nominations.
"Declining unanimous consent agreements, doubling the number of amendments they're demanding, using special procedures to try and divide bills into multiple sections. They have a number of options available to them."
Republicans wouldn't disclose exactly how they will respond but made clear they won't stand idly by.
"I think this senator summed it up nicely," said Don Stewart, a senior aide to McConnell, pointing to a quote from then-Sen. Barack Obama during the 2005 standoff.
"Everyone in this chamber knows that if the majority chooses to end the filibuster, if they choose to change the rules and put an end to democratic debate, then the fighting, the bitterness, and the gridlock will only get worse," Obama said then.
Both Reid and McConnell were asked on Sunday about being on different sides of the argument in 2005.
Reid said the change he is proposing is "very minimal," applying only to Cabinet and executive branch positions.
"This is not judges, this is not legislation -- this has allowing the people of America to have a president who can have his team in place," Reid said on NBC's "Meet the Press." "This is nothing like what went on" in 2005.
Appearing later on the same program, McConnell seemed to tamp down some of the acrimony. Three days after saying Reid would be remembered as the worst Senate leader ever, McConnell said Reid was "a reasonable man, he's a good majority leader."
Who would come out worse if crisis not resolved?
A Democratic leadership aide said Republicans have more to lose in a slowdown than Democrats do.
"The conundrum -- for Republicans, not for Democrats -- is whether they can bear shutting the Senate down over a rules change that allows simple nominations to be confirmed, and therefore also end support for popular items like immigration reform, the farm bill, and investments in education, infrastructure and health research," said a Democratic leadership aide. "It may play well with their base, but the outrage is certain to fall on deaf ears with the Hispanic community, farmers, teachers, parents and students."
Two other veterans of the 2005 fight, which was resolved after a bipartisan "Gang of 14" senators reached a compromise, expressed hope this standoff would be resolved, too, without a change in filibuster rules.
At that time, Bob Stevenson was a top aide to then-Majority Leader Bill Frist of Tennessee. He described the threat then as an "action-forcing mechanism" that resulted in the creation of the "Gang of 14."
Stevenson warned if Reid employs the nuclear option now it would "dramatically change the very nature and culture of the Senate, spilling over into already poisonous partisan debates down the road and making resolution of those issues even more difficult."
He said he is hopeful, but not optimistic, that when senators meet Monday night, the seriousness of the potential consequences will lead to "cooperation and compromise."
A Democratic aide, who was also at the center of the 2005 fight and who asked not to be identified, said he hopes when the senators meet privately maybe "some of the more seasoned veterans of these battles" will come forward and say "'Isn't there a way to resolve this?' That's what happened in 2005."
McConnell said Sunday that he hoped an ugly fight can be avoided, as it was in 2005 when "cooler heads prevailed."
"We knew it would be a mistake for the long-term future of the Senate and the country. That's what I hope is going to happen here, David," McConnell told NBC's David Gregory. "We have an opportunity to pull back from the brink in this meeting we're going to have of senators in the Old Senate chamber Monday night. I hope we'll come to our senses and not change the core of the Senate."