Republicans capture control of the House after falling short of midterm expectations.

In an interview over the weekend on ABC News, Ms. Pelosi said, “I don’t have any plans to step away from Congress.” She makes little effort to mask her disdain for her potential successor, Mr. McCarthy, and his ability to serve as House speaker, a powerful post that is second in the presidential line of succession.

“No, I don’t think he has it,” Ms. Pelosi said on CNN over the weekend, when asked if Mr. McCarthy had what it takes to be speaker. “But that’s up to his own people to make a decision as to how they want to be led or otherwise.”

Democratic control of the Senate ensures that whatever agenda Mr. McCarthy and the Republicans push through is likely to be dead on arrival in the upper chamber. Senator Chuck Schumer of New York, the Democratic majority leader, has called his chamber the “firewall” against Republican priorities.

With a Senate under Democratic control, Mr. Biden will continue to be able to have his cabinet and judicial appointments confirmed, including any potential Supreme Court vacancies. In the House, Republicans will now have broad subpoena power to initiate investigations into the Biden administration, and Mr. McCarthy has signaled that they are eager to do so.

The White House began making plans months ago for an onslaught of Republican investigations, hiring Richard A. Sauber, a white-collar defense lawyer, as “special counsel to the president” to oversee the response to subpoenas and other oversight efforts.

The relationship between Mr. Biden and Mr. McCarthy as their parties’ most powerful leaders in Washington got off to an inauspicious start.

Mr. Biden told reporters that he called Mr. McCarthy right after the election and said, “If you win the majority, congratulations. But congratulations — so far, you’ve made some gains.” Mr. McCarthy characterized it differently on Fox News: “He congratulated me, so for anyone who thinks we didn’t win the majority, Joe at least believes we did as well.”