Senate Democrats on Tuesday enlisted Janet L. Yellen, the Federal Reserve chairwoman, as an expert witness against Republican plans to roll back postcrisis financial regulations.

In testimony before the Senate Banking Committee, Ms. Yellen said repeatedly that the new safeguards were effective and that regulation was not impeding economic growth.

Senator Sherrod Brown of Ohio, the ranking Democrat on the committee, asked Ms. Yellen whether regulation had reduced the risk of financial crises.

“I believe the financial system is much more resilient than it was,” she said.

Are businesses unable to get loans? No, Ms. Yellen responded.