Several Republicans seized on her answer and then pressed her to agree to strip the agency of its power to ban such practices if she did not see any immediate need to use them. But Ms. Warren balked, saying the power to ban practices was one important tool the new agency needed to fix “a broken consumer credit system.”

Expanding on her vision for the agency, Ms. Warren said she saw it as a “cop on the beat” that would regulate lending institutions, seek to simplify loan documents to make them more understandable and guard against fraud and abuse.

Republicans have unsuccessfully tried to chip away at the authority and structure of the agency by proposing a commission instead of a single director to oversee it, among other ideas. They signaled Thursday that they want to reduce the agency’s powers by challenging the salaries of some staff members, the justification for its budget decisions, its authority to look into certain types of financial institutions and practices, and a number of other organizational issues.

Representative Ann Marie Buerkle, Republican of New York, said she worried that an overly intrusive consumer agency would drive up the cost of compliance by lenders and make it harder for small businesses and others to get affordable credit.

Ms. Warren said that the large banks and lenders hired “armies of lawyers” who produced loan documents that were indecipherable to many Americans. “We need some pushback. We’re the voice on behalf of the customers, the American families,” she said.

She said that the agency’s regulation of the industry should help to lower the costs of credit to Americans, rather than raise it, and make borrowers “a little more secure.”