Senate Democrats have rekindled their threats to go "nuclear" and unilaterally change the rules to make it tougher for the minority party to filibuster after Republicans blocked two key White House nominations last week.

And with Democrats accusing Republicans of breaking a handshake deal to temper their use of the filibuster, many around Washington believe Democrats aren't bluffing.

"I do" think Democrats are serious, said Norm Ornstein of the conservative-leaning American Enterprise Institute. "The frustration [among them] is growing."

President Obama has struggled for years to overcome Republican opposition to his nominations for top government or judicial posts, particularly for the D.C. Circuit Court -- considered the nation's most powerful bench after the Supreme Court.

And last week Senate Republicans again held together while they blocked the nominations of Rep. Mel Watt, D-N.C., to head the Federal Housing Finance Agency and Patricia Millett to the D.C. Circuit Court of Appeals. Both got a majority of votes in the 100-seat chamber but failed to get the 60 voted needed to break a filibuster.

The votes essentially blew up a fragile months-old leadership truce in which Senate Majority Leader Harry Reid, D-Nev., promised not to change the chamber's rules by a majority vote -- the so-called nuclear option -- to make it harder for the minority Republicans to block presidential nominations. In exchange, Reid said Senate Minority Leader Mitch McConnell, R-Ky., agreed to filibuster judicial nominations only in "extraordinary circumstances."

Democrats, as well as many political experts, say neither Watt nor Millett was a controversial pick.

Watt, a soft-spoken 20-year House veteran, rarely if ever has been portrayed as a partisan ideologue. He has served for years on a committee dealing with housing matters, which supporters said made him an ideal candidate to head the federal agency that oversees mortgage lenders Fannie Mae and Freddie Mac.

And Millett's supporters say she is a highly respected lawyer who has argued more than 30 cases before the Supreme Court and has served in Democratic and Republican administrations.

Reid called the filibusters "unprecedented obstruction." And Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., said the Republicans' action was so uncalled for that it may be time for Democrats to invoke the nuclear option.

"No argument has been lodged against [Millett] that would rise to the level of an extraordinary circumstance," Leahy said. “If the Republican caucus finds that despite her stellar legal reputation and commitment to her country that somehow a filibuster is warranted, I believe this body will need to consider anew whether a rules change should be in order."

Even Vice President Joe Biden said such a move is "worth considering."

But Republicans say the filibuster — which doesn't exist in the House — is one of the only tools at their disposal to ensure the controlling Democrats don't run roughshod in the Senate. Any attempt to diminish its power, they say, would compromise a delicate system of checks and balances.

As for the nominees they blocked, Republicans said that while they respect Watt, he simply doesn't have the work expertise for the job. And the Millett nomination, they said, was a blatant attempt by Obama to "stack" the D.C. Circuit Court with liberal judges.

But Ornstein said the GOP's claim that the nominees don't deserve a final up-or-down vote is a difficult argument to make.

"There was an implicit agreement [between Reid and McConnell] that on appeals court judges, if there were filibusters, it would be confined to people who were viewed by Republicans as unqualified or outside the mainstream," Ornstein said. "Now you've got Republicans doing it on a very regular basis, and that has ratcheted up the stakes here significantly."

Reid has been coy as to whether he would change Senate rules to make it more difficult for Republicans to filibuster, saying he won't talk about "hypotheticals."

And Senate Minority Whip John Cornyn, R-Texas, characterized the Democrats' threat as idle chatter.

"Senator Leahy knows that someday there'll be another Republican president, and the same rule ... will apply to" them, he said. "So I don't really take this threat to the nuclear option very seriously when it comes to the judicial nominations."

Bill Galston of the liberal-leaning Brookings Institution said he wouldn't put it past Reid to change the rules.

"Nerves are as frayed on the Hill as they've been since I've been in Washington," said Galston, a longtime congressional expert. "For many Democrats the situation before them [when the Watt and Millett nominations were blocked] was very far from the sort of situation that the phrase 'extraordinary circumstances' by any stretch could be applied" to presidential nominations.

When asked if McConnell broke the "spirt and the letter" of his deal with Reid to limit his use of the filibuster, Galston said; "It's hard to do anything except give an affirmative answer to that question."

Two more Obama nominees for the D.C. Circuit Court have cleared the Senate Judiciary Committee and are awaiting final votes in the full Senate. Republicans leaders haven't said if they will filibuster the nominations, which Reid is expected to bring to the floor in the coming weeks.

But Sen. Lindsey Graham, R-S.C., already has threatened to hold up all White House nominations until survivors of last year's deadly attacks on a U.S. diplomatic outpost in Benghazi, Libya, appear before Congress.

Ornstein said if Republicans block even one of the two remaining D.C. Circuit Court nominations, then Democrats will feel "almost inexorable" pressure to change the rules.

"If you block all three, or you only let through one of the three, where it's clear you're using [the filibuster] simply to try to tilt the balance of the court against the president's legitimate ability to fill vacancies, then I think you'll see a change in the rules," Ornstein said.