“I’m proud to endorse Donald J. Trump for president,” Ms. Palin said in a statement provided by his campaign.
Her support is the highest-profile backing for a Republican contender so far.
“I am greatly honored to receive Sarah’s endorsement,” Mr. Trump said in a statement trumpeting Mrs. Palin’s decision. “She is a friend, and a high-quality person whom I have great respect for. I am proud to have her support.”
Mrs. Palin, who is to appear alongside Mr. Trump at a rally on the Iowa State University campus in Ames late Tuesday afternoon, could amplify the news media-circus aspects of Mr. Trump’s candidacy: Like him, she is a reality-TV star accustomed to playing to the cameras and often accused of emphasizing flash over substance.
But Mrs. Palin, who despite her waning visibility within the Republican Party retains a sizable following, provides Mr. Trump with valuable new currency at a moment when he is being attacked over his conservative bona fides by Senator Ted Cruz of Texas, with whom Mr. Trump is neck-and-neck in the Iowa polls.