Donald J. Trump is parting ways with his campaign manager, Corey Lewandowski, a move that comes as the presumptive Republican nominee faces challenges as he moves toward the general election.

“The Donald J. Trump Campaign for President, which has set a historic record in the Republican primary having received almost 14 million votes, has today announced that Corey Lewandowski will no longer be working with the campaign,” the campaign spokeswoman, Hope Hicks, said in a statement. “The campaign is grateful to Corey for his hard work and dedication and we wish him the best in the future.”

Mr. Trump had faced increasing concerns from allies and donors, as well as his children, about the next phase of the campaign as he pivots toward a general election.

One person briefed on the moves, who asked not to be identified, said the campaign had long planned adjustments to the needs of a general election campaign.

Mr. Trump has recently seen his standing dropping in recent national polls, and is now facing a barrage of ads by Hillary Clinton and her allies.

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