Donald J. Trump plans to name his former rival, Robert F. Kennedy Jr., and Tulsi Gabbard, a one-time Democrat, as honorary co-chairs of a presidential transition team that will help him select the policies and personnel of any second Trump administration, according to a campaign senior adviser.

Mr. Kennedy, who ended his independent campaign for president and endorsed Mr. Trump on Friday, described his transition role briefly in an interview that aired Monday.

“I’ve been asked to go on the transition team, you know, and to help pick the people who will be running the government,” Mr. Kennedy told Tucker Carlson, the former Fox News host, in an interview posted on X.

Both Mr. Kennedy and Ms. Gabbard spent most of their public life as progressive Democrats. Only four months ago, Mr. Trump was calling Mr. Kennedy a “Radical Left Lunatic” who was “far more LIBERAL than anyone running as a Democrat.” Trump allies pushed stories about Mr. Kennedy’s record of supporting abortion rights and far-left environmentalism as they tried to make his independent candidacy less appealing to Trump voters.

Ms. Gabbard, a former congresswoman who left the Democratic Party after her 2020 presidential run and who endorsed Mr. Trump on Monday, has been helping with the informal debate preparation sessions Mr. Trump has held in recent weeks.

It’s unclear what exactly she and Mr. Kennedy will do in their transition roles, but they will join Mr. Trump’s sons, Donald Trump Jr. and Eric Trump, as well as his running mate, Senator JD Vance, as honorary chairs of the transition.

Brian Hughes, a Trump campaign senior adviser, said that they are “proud that Robert F. Kennedy Jr. and Tulsi Gabbard have been added to the Trump/Vance Transition team. We look forward to having their powerful voices on the team as we work to restore America’s greatness.”

Mr. Kennedy and Ms. Gabbard did not respond to requests for comment on Tuesday.

Chris Cameron contributed reporting.