Washington (CNN) President Donald Trump questioned the politics of his decision to finally acknowledge former President Barack Obama was born in the US, which he did late during the campaign in 2016, according to a source close to the White House.

The source said that shortly after he made the statement, Trump told aides that he would have done better in the polls had he continued to stand his ground on the birth certificate issue.

Trump has continued to question the legitimacy of Obama's birth certificate during private conversations in recent months, The New York Times reported Tuesday, citing advisers who discussed Trump's statements.

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One sitting US senator quoted by the Times, "who listened as the President revived his doubts" about the issue, "chuckled" when speaking about what was said. Trump "has had a hard time letting go of his claim that Mr. Obama was not born in the United States," the senator, who "asked not to be named to discuss private conversations," told The New York Times.

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