Ms. Whitman was a leading fund-raiser for Mr. Romney’s 2008 presidential campaign and was a chairwoman of Gov. Chris Christie’s presidential finance team this year. But after Mr. Christie withdrew from the campaign and endorsed Mr. Trump, Ms. Whitman excoriated the New Jersey governor for what she said was an “astonishing display of political opportunism.”

On Tuesday, Ms. Whitman said she had not spoken to Mr. Christie since he endorsed Mr. Trump, and pointedly noted that she had not changed her view of his decision.

Ms. Whitman, who said she would remain a Republican, brings with her a considerable network of contributors, some of whom she said were open to giving to Mrs. Clinton. She said she was willing to campaign for Mrs. Clinton, said she would do her best to gather checks for her campaign and indicated she would personally give to both Mrs. Clinton and her affiliated “super PACs.” An aide to Ms. Whitman said she would personally give at least an amount in the “mid-six figures” to the Clinton effort.

While Democrats openly appealed at their convention last week to Republicans uneasy with Mr. Trump, Mrs. Clinton and her top supporters have been making a similar cross-party pitch in private since before the Democratic nomination fight even came to its conclusion.