Ms. Noem, the governor of South Dakota, would play a major role in carrying out the president-elect’s promises to crack down on the border and deport millions of people.

President-elect Donald Trump is planning to nominate South Dakota Republican Gov. Kristi Noem to serve as the secretary of the Department of Homeland Security, according to four sources familiar with the decision.

In Trump’s second administration, Noem would join several former Trump officials who were the architects and biggest defenders of his hard-line immigration policies.