WASHINGTON — In May, just weeks after becoming secretary of state, Mike Pompeo returned to Capitol Hill for defiant and surprisingly heated exchanges about diplomacy with his former Democratic colleagues.

“I’ll take a back seat to no one with respect to caring about and protecting the people,” he said of diplomatic security in one blistering answer during a hearing in the House, where he had served for six years as a Republican congressman from Kansas. Before the Senate, he dismissed another question from a Democrat as “bizarre” and “outrageous.”

At the time, Mr. Pompeo could afford to dismiss minority Democrats who had little to no sway over the State Department. That will change in January, when Democrats will take over the House and have subpoena and budgetary powers on the Foreign Affairs and Appropriations Committees — both of which serve as a check on the State Department.

Even in the Senate, where Republicans retain control, Democrats on the Foreign Relations Committee can continue to hold up nominations at the State Department, which is struggling to fill its ambassador ranks and other senior diplomatic posts.