(CNN) -- The bitter fight over nominations resumes Thursday as Senate Republicans appear poised to try and block two of President Obama's appointments, possibly triggering an angry backlash from Democrats.
One is the President's pick to be a judge on the highly influential U.S. Court of Appeals in Washington, D.C. The other is a Democratic congressman the President wants to head the federal agency that oversees the mortgage giants Fannie Mae and Freddie Mac.
If Republicans succeed in blocking the two, Democrats might carry out a threat to change Senate rules over GOP objections, the so-called "nuclear option," to prevent future filibusters of presidential appointees.
Senate Majority Leader Harry Reid said Wednesday he was "cautiously optimistic" Republicans would approve Patricia Millett to the appeals court, which is often considered second in importance to the Supreme Court. He declined to "get into hypotheticals" before the votes take place about whether he'd push for the rules changes.
Republican aides were doubtful -- although not fully confident -- that at least five GOP senators would break with their party to support Millett. They said Sens. Susan Collins of Maine and Lisa Murkowski of Alaska might vote with Democrats to break the filibuster. If they did, that would still leave Democrats three votes shy of the 60 needed to break the filibuster.
Senior Democratic aides seemed genuinely unsure of whether they would have enough votes to move Millett's nomination, unusual on the eve of a key vote.
Since Millett was chosen a few weeks ago, Republicans have said Democrats and the President are trying to change the balance on the court, which currently has four judges who were appointed by Democratic presidents and four judges who were appointed by Republican presidents.
While there are currently three vacancies on the 11 seat bench, Republicans argue its workload is lighter than some other circuits and they would like to see vacancies filled elsewhere first.
"I think we're not going to allow that court to be stacked," said Sen. Lindsey Graham, R-South Carolina. "She'd be a fine judge, just put her somewhere else."
"It's clear they are trying to pack the court so they can have their judges pass legislation they can never get through the Congress," said Sen. Orin Hatch, R-Utah. "I don't think Republicans are going to roll over on this."
Democrats argue the Republican effort to block Millett is a transparent attempt to preserve conservative influence on the appeals court.
"If this goes down, it's just politics," said Sen. Dianne Feinstein, D-California, who is pushing for Millett's confirmation. "This woman is superbly qualified."
Reid has been frustrated for years about Republicans slow-walking nominations and in the past has threatened the rules change, which Republicans warn would destroy cooperation in the chamber. But he's held back despite pressure from many members and liberal interest groups to implement the controversial new rules that would allow approval of nominees with just a 51-vote majority.
"The pressure to change the rules will be almost insurmountable" if Republicans block Millett, Sen. Patrick Leahy, D-Vermont, who chairs the Judiciary Committee said Tuesday.
One top Republican leadership aide said he doubted Reid would change the rules. The aide said Reid also is under pressure to preserve the ability of Democrats to filibuster the nominees of future Republican presidents and he wouldn't want to throw that away over this appointment.
The fight over Millett comes as part of a broader Democratic push this week on a series of Obama appointments. So far each of the executive branch nominees has been approved. But now there are doubts about Millett and Rep. Mel Watt, D-North Carolina, who the President picked to head the Federal Housing Finance Agency, which oversees the controversial mortgage entities Fannie Mae and Freddie Mac. Republicans have argued Watt is not independent enough and lacks the expertise to manage the complex agency.
Democratic Senate aides conceded privately that is unlikely Watt will get the 60 votes he needs for confirmation.
One aide said if both Millett and Watt are defeated, rank and file Democratic senators would be more likely to push for the nuclear option, something Democratic leaders had not initially envisioned would be the result of this week's voting.