WELLESLEY, Mass. — Hillary Clinton never named him, but she excoriated him.

Speaking at the Wellesley College commencement on Friday, she alluded again and again to Donald J. Trump, the man who defeated her for president. She referred to an “assault on truth and reason,” criticized Mr. Trump’s proposed budget, and drew parallels to Richard Nixon and his ultimate fate: being forced from office under a threat of impeachment.

These days, she said, those in power are discarding reality in favor of inventing their own “alternative facts,” which, she warned, could lead to the beginning of the end of a free society.

“That is not hyperbole,” she said. “It is what authoritarian regimes throughout history have done. They attempt to control reality — not just our laws and rights and our budgets, but our thoughts and beliefs.”

The graduates at Wellesley, Mrs. Clinton’s alma mater, cheered her thunderously and treated her like a returning hero — if not one who had conquered, at least one who in their eyes remained unbowed.