ATLANTA — President Trump disparaged Stacey Abrams, the Democratic nominee for governor of Georgia, in ambiguous and unusually personal terms on Thursday, warning that “her past” left her “not qualified to be the governor.”

Mr. Trump did not elaborate and offered no evidence for his assertion, which seemed to be a potential preview of the political message he will deliver on Sunday, two days ahead of the election, at a Georgia rally for Brian Kemp, Ms. Abrams’s Republican rival.

But the decision of the president, who has been criticized for inflammatory language, to invoke Ms. Abrams’s background so broadly was a distinct escalation in his attacks on her bid to become the first black woman to be elected governor in the United States. Ms. Abrams, a former Democratic leader of the Georgia House of Representatives, has staked out an array of liberal positions during her campaign, but her tenure in the Legislature has drawn measured praise from the Republicans who led the State Capitol.

Mr. Trump had previously attacked Ms. Abrams on Twitter, but he had not made any overt references to her background until Thursday.