Representative Tulsi Gabbard of Hawaii endorsed the impeachment inquiry against President Trump on Friday, dropping longstanding opposition that she had affirmed as recently as two days ago.

Her announcement means the 19 Democratic presidential candidates unanimously support the inquiry. Ms. Gabbard had been the only holdout after the White House released a reconstruction of a July phone call in which Mr. Trump urged the president of Ukraine to investigate former Vice President Joseph R. Biden Jr. and his son.

“Up to this point, I have been opposed to pursuing impeachment because it will further divide our already badly divided country,” Ms. Gabbard said in a statement from her congressional office. “However, after looking carefully at the transcript of the conversation with Ukraine’s president, the whistle-blower complaint, the inspector general memo and President Trump’s comments about the issue, unfortunately, I believe that if we do not proceed with the inquiry, it will set a very dangerous precedent.”

She added: “If we allow the president to abuse his or her power, then our society will rot from top to bottom. We will turn into a banana republic, where people in positions of power — from the president all the way down to the traffic cop — will feel it’s O.K. to abuse their power with no consequences.”