Senate Republicans on Monday night once again blocked a key White House judicial nominee, heating up calls from Democrats who say it's time to change the rules to make it more difficult for the minority party to filibuster.

Robert Wilkins' nomination to fill one of three vacant seats on the U.S. Court of Appeals for the District of Columbia Circuit — considered the nation's second most powerful bench behind the Supreme Court — fell six votes shy of the 60 needed to overcome a GOP filibuster.

Wilkins is the latest in a growing list of Obama administration nominees, and the third of the three nominees for the D.C. Circuit, whom Republicans have rejected in recent weeks.

President Obama said he was "deeply disappointed" by the Senate Republicans' action.

They "have once again refused to do their job and give well-qualified nominees to the federal bench the yes-or-no votes they deserve," the president said.

The president complained that his immediate predecessor, President George W. Bush, had four of his six nominees to the D.C. Circuit confirmed.

"I am fulfilling my constitutional responsibility, but Congress is not," he said.

Sen. Chuck Grassley of Iowa, the senior Republican on the Senate Judiciary Committee, said he opposed Wilkins' nomination on the grounds the D.C. Circuit's caseload has significantly waned in recent years, rendering the vacant post — and its estimated $1 million annual cost — unnecessary.

Grassley added that Democrats, who were in the minority in 2006, used the same rationale to block a Bush nominee to the court.

"The other side has run out of legitimate arguments in support of these nominations," Grassley said. "That is why they are grasping at straws."

Some Democrats say the complexity of the D.C. Circuit's caseload warrants a full 11-member bench. But Grassley and other Republicans say the argument doesn't hold up to scrutiny.

But Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., said Republicans were eager to fill vacancies on the court when Republicans have occupied the White House. He said Wilkins has the American Bar Association's highest rating and is being opposed by Republicans only because he was nominated by Obama.

Leahy also said it's the GOP's "unprecedented level of obstructionism" that is leading many Democrats to push to use the so-called "nuclear option" — a parliamentary move to unilaterally change the chamber's rules to make it more difficult for the minority party to block presidential nominees.

"This kind of delay for the sake of delay, this kind of treating this president different than other presidents — that's why there's momentum for a change in our rules," he said.

Sen. Ben Cardin, D-Md., said he supports changing Senate rules to allow all judicial nominees receive a straight "up-or-down" majority vote.

"I think the use of the filibuster has been abused," Cardin said. "I am frustrated as any ... of not having [an] up-or-down vote."

Rep. Marcia L. Fudge, D-Ohio, who is the chairwoman of the Congressional Black Caucus, pressed her Democratic colleagues in the Senate to take "all necessary steps" to find a way to overcome GOP filibusters.

"I continue to be appalled by Senate Republicans' indignant refusal to confirm President Obama's nominees," she said. "All that matters to Senate Republicans is that they score political points."

Several outside groups also are pushing Democrats to follow through on their threat.

"Those intent on preserving the norms and traditions of the U.S. Senate should embrace this [filibuster] reform effort," said a statement released after the vote from Fix the Senate Now, a coalition of progressive and labor groups. "It is the Republicans’ filibuster en masse and without regard to the individual merits of nominees that already represents a radical departure from Senate history."