The Senate blocked President Obama’s nominee to head the Consumer Financial Protection Bureau, as filibustering Republicans who oppose the very powers of the new agency successfully challenged one of the administration’s main responses to the financial crisis.

The nomination of Richard Cordray was rejected after Democrats failed to achieve the 60 votes they needed to move his nomination forward. The vote was 55 yes, 45 no.

Senator Mitch McConnell of Kentucky, the Republican leader, said his party had made clear for months that it would not approve a leader for the watchdog consumer agency until the law that established it was amended.

Until three changes are made, he said, “We won’t support a nominee for this bureau — regardless of who the president is.”

The changes they want, he said, would put a board of directors in charge of overseeing the bureau, subject the agency to the Congressional appropriations process — thereby giving lawmakers more sway over its policies — and give other financial regulatory agencies a check on its rules.

Senator Sherrod Brown of Ohio, a Democrat, said that opponents’ “first loyalty is to Wall Street banks.”

Previous opposition from Republicans led to the withdrawal of Elizabeth Warren from consideration for the post. She is a Harvard law professor who had been the driving force behind the agency’s creation and who is now a Democratic candidate for the U.S. Senate in Massachusetts.