President-elect Donald J. Trump on Wednesday chose Tulsi Gabbard, a former Democratic congresswoman who became one of his most enthusiastic backers, to serve as the director of national intelligence.

Ms. Gabbard, a lieutenant colonel in the Army Reserve who served in Iraq, has been a longtime critic of the foreign policy establishment. Her nomination is another sign that Mr. Trump intends to give top foreign policy jobs to supporters who are deeply skeptical of the effectiveness of U.S. military intervention abroad.

In a statement, Mr. Trump praised Ms. Gabbard and said she would bring “a fearless spirit” to the intelligence agencies and secure “peace through strength.” The statement announcing the nomination said she was a former Democrat who had joined the Republican Party “because of President Trump’s leadership and how he has been able to transform the Republican Party, bringing it back to the party of the people and the party of peace.”

Along with John Ratcliffe, Mr. Trump’s choice to lead the C.I.A., she would be a top intelligence adviser to the White House. She would oversee 18 spy agencies and would be responsible for preparing the President’s Daily Brief, a written intelligence summary assembled each morning. In his first administration, Mr. Trump did not often read the written summary. But he held in-person intelligence briefings, often twice a week or more, engaging his briefers on world affairs, at least on topics that interested him.

Ms. Gabbard left the Democratic Party after a failed run for the presidential nomination in 2020. Her subsequent enthusiasm for Mr. Trump made her a celebrity among his supporters.

She was briefly considered by Mr. Trump as a possible running mate, before he winnowed the field down to other finalists. Ms. Gabbard helped Mr. Trump prepare his attacks against Vice President Kamala Harris while he was preparing for his September debate.