WASHINGTON — Vice President Mike Pence has hired Jon Lerner, a veteran Republican pollster who is a deputy for the United Nations ambassador, Nikki R. Haley, as his national security adviser, a spokeswoman for Mr. Pence said on Thursday evening.

Mr. Lerner will continue to work with Ms. Haley while advising Mr. Pence “full time,” according to Alyssa Farah, the vice president’s spokeswoman.

The unusual arrangement, reported on Thursday by The Washington Post, gives Mr. Pence and Ms. Haley a single key voice on national security.

The move comes during a week of major upheaval for President Trump’s own national security team. John R. Bolton took over as national security adviser on Monday and immediately began shaking up the ranks, pushing out Thomas P. Bossert, the president’s chief adviser on homeland security, on Tuesday. On Thursday, Mike Pompeo, the C.I.A. director whom Mr. Trump has nominated for secretary of state, underwent a Senate confirmation hearing.