Republicans recoiled Wednesday night and Thursday morning over a bill some Democrats are proposing to pack the Supreme Court, calling it an "assault" on judicial independence. 

But they also predicted that the move would prove unpopular and help Republicans in the midterm elections. 

"Democrats are launching a full assault on the independence of the federal judiciary. Republicans will stop them," Rep. Ken Buck, R-Colo., said in a tweet. 

"This is such a fantastic gift to the NRSC," added Matt Whitlock, a former staffer at the National Republican Senatorial Committee (NRSC). "And making it a bill instead of a Senate rules process means we get to get moderate House D's on the record on it too. Just a fantastic turn of events."

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Sen. Ed Markey, D-Mass.; House Judiciary Committee Chairman Jerrold Nadler, D-N.Y.; Rep. Hank Johnson, D-Ga.; and Rep. Mondaire Jones, D-N.Y.; will announce the bill at 11:30 a.m. Thursday in a press conference on the steps of the Supreme Court. 

The legislation would expand the Supreme Court to 13 seats. There are currently nine seats on the court. That number has remained the same since 1869. 

Some Democrats have been loudly calling for the expansion of the Supreme Court after former President Donald Trump had three nominees confirmed during his presidency. They decry some of the rulings the court has issued, accusing the justices of being biased toward Republicans.

"Republicans packed the court when Mitch McConnell held Merrick Garland’s seat open nearly a year before an election, then confirmed Amy Coney Barrett days before the next election. Disarming the Court’s radical right-wing majority would correct this injustice," Jones said Thursday. 

But court-packing is not filling empty seats on the Supreme Court, as Jones says. It instead means adding seats to the Supreme Court in order to change the results of its rulings. The term was first coined when it was unsuccessfully attempted by former President Franklin Delano Roosevelt. 

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Jones added Wednesday that expanding the Supreme Court "is infrastructure." 

"He doesn't even mean this is a parody, which is what makes it great. His moderate Democrat colleagues facing tough races next year won't thank him for this stunt," Whitlock responded. 

Democrats' court-packing proposal is in contrast to comments made by Justice Stephen Breyer last week opposing court-packing. 

"My experience of more than 30 years... as a judge has shown me that once men and women take the judicial oath they take that oath to heart. They are loyal to the rule of law, not to the political party that helped to secure their appointment," Breyer said. "These considerations convince me that it is wrong to think of the court as just another political institution and it is doubly wrong to think of its members as junior league politicians."

Breyer added: "Structural alteration motivated by the perception of political influence can only feed that latter perception, further eroding that trust. There is no shortcut."

Republicans said that they plan to fight hard against the Democrats' effort. The conservative Judicial Crisis Network (JCN) is launching a $1 million ad campaign, spanning three weeks, opposing the court-packing bill. The ads will air on national cable and in the Washington, D.C., area.

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"The Left and the liberal dark money groups that support them won't stop agitating to pack the Court," JCN President Carrie Severino tweeted. "They won't be satisfied with a report from a commission. They want to pack the Court with politicians in robes who will advance their radical agenda."

Some Republican senators also weighed in. 

"Packing the court is an act of arrogant lawlessness. Those behind this effort spit in the face of judicial independence," Sen. Mike Lee, R-Utah, said. 

"Here are some facts: The Supreme Court isn’t supposed to be America’s super-legislature; Democrats don’t have some historic mandate in a 50-50 Senate to nuke the Court; the progressive activists who wrote this bill are high on their own supply; and a whole bunch of sane Democrats are quietly praying this thing dies," Sen. Ben Sasse, R-Neb., said. "The court-packing bill is delusional."

Added Sen. Tom Cotton, R-Ark.: "Packing the Supreme Court would destroy the Supreme Court. The Democrats will do anything for power."

Sen. Steve Daines, R-Mont., meanwhile said that he will reintroduce a resolution against court-packing. This follows constitutional amendments proposed earlier this week by Rep. Mike Gallagher, R-Wis., and earlier this year by Rep. Dusty Johnson, R-S.D., that would limit the number of justices to nine. 

"Packing the Supreme Court is an attack on our Montana way of life," Daines said. "It is a blatant power grab by the Democrats to pave the way for the Left’s radical agenda. We cannot let this happen. We must protect our freedoms and the future of our country."

"It's taking over an entire branch of government, the judicial system, and it's packing them," House Minority Leader Kevin McCarthy, R-Calif., said in a radio interview. "It goes against everything we believe as Americans... It goes against what Joe Biden back in the day when he was a senator would say ever to do." 

"This should scare every single American, regardless of where you stand politically," McCarthy added in an interview on "Mornings with Maria." "This just goes to show how far the Democrat Party has moved. There almost are no longer common-sense or moderate Democrats elected... It's overtaking a branch of government simply to have your control over a nation. It must be the scariest thing I've ever heard them do." 

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Other high-profile Republicans, including Senate Minority Leader Mitch McConnell, R-Ky., have yet to weigh in. Several moderate Democrats, including Sens. Joe Manchin, D-W.Va., and Kyrsten Sinema, D-Ariz., Rep. Jared Golden, D-Maine, and Josh Gottheimer, D-N.J., also have not commented on the bill. 

But the legislation is highly unlikely to pass. 

Democrats can only lose two votes in the House and still be able to pass a bill. And in the Senate, they would need to get rid of the legislative filibuster to pass their court-packing bill. Manchin and Sinema have said they will not do that. Manchin also previously said he opposes court-packing. 

Fox News' Marisa Schultz contributed to this report.