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The Connecticut independent, who caucuses with the Democrats, said lawmakers “must constantly strive for a balance that satisfies the legitimate demands of both branches.”

Mr. Lieberman, who has held hearings on czar accountability, nevertheless said the Vitter amendment was overly broad.

Also opposing the amendment was Sen. Charles E. Schumer, New York Democrat. In a floor speech, he called the proposal “a poison pill designed to handcuff the president’s ability to assemble a team of top-flight advisers and aides.”

“Now is the time to move forward. … It is not the time or place to relitigate old and, frankly, silly political battles about so-called czars,” Mr. Schumer said of the underlying legislation.

The measure to streamline judicial nominations eventually passed.

Although Mr. Obama is not alone in his generous use of policy czars, some specialists argue that his reliance has been unprecedented in size and scope.

Obama has more czars than any past president, hands down,” said Mark J. Rozell, a George Mason University professor who is co-authoring a book on White House czars. He argues that czars violate the spirit of the Constitution’s separation of powers.

“Many of them make policy, regulatory and spending decisions, but they are not confirmed, and they are not subject to testimony before Congress,” Mr. Rozell said. “The president’s critics are right to challenge the use of czars for these reasons, regardless of party ID.”

Russ Feingold and Robert C. Byrd were among Senate Democrats who raised constitutional questions, but the czar issue has been a particular flash point among Republicans.

Those critics seized on missteps by some of the higher-profile czars, such as the former “green jobs czar” Van Jones, whose ties to the Sept. 11 “truther” movement led to his resignation, and former “climate czar” Carol M. Browner’s previous association with an international socialist group.

More recently, congressional Republicans have asked “car czar” turned “manufacturing czar” Ron Bloom to clarify remarks he made to an oversight committee about the government’s auto industry bailout.

Mr. Bloom had denied telling colleagues that he did it “all for the unions,” as his predecessor claimed in a book.

Although Mr. Obama has eliminated some of the more contentious posts, he continues to defend his prerogative to appoint czars. In April, lawmakers attached to a budget bill a provision to strip federal funds for four czars — three of whom already had left their posts. The president said in a signing statement that he would ignore the language.