SALEM, Ore. — Gov. John Kitzhaber, long regarded as a wily survivor of Oregon politics, resigned Friday amid a spiraling crisis that included a criminal investigation of the role that his fiancée played in his administration and crumbling support from his Democratic Party colleagues.

It was a steep and rapid fall for Mr. Kitzhaber, 67, a former emergency room doctor who won an unprecedented fourth term as governor in November. His resignation means that Kate Brown, the Oregon secretary of state and a fellow Democrat, will become governor, in accordance with the succession plan in the state Constitution.

Even during the recent election, Mr. Kitzhaber had been plagued by questions about his fiancée, Cylvia Hayes, with whom he lives in the governor’s mansion, and whether she had violated ethics rules or criminal laws in advising him about clean energy issues while serving as a consultant on the topic. Before November’s election and after, he repeatedly denied any wrongdoing by Ms. Hayes, 47, or his office, and pledged cooperation in the various inquiries, including one initiated this month by the state’s attorney general, Ellen Rosenblum, also a Democrat, which could result in criminal charges.

But in the last few days, some senior Democrats in the heavily Democratic state abandoned him and called for his resignation, piling on with some Republicans, who had criticized him as an ineffective leader even before the scandals. Ms. Brown, 54, the secretary of state, was among those who distanced herself, releasing a statement on Thursday describing what she said was a “bizarre” meeting she had had with Mr. Kitzhaber — saying that he had asked her to rush back to Oregon from a conference in Washington, D.C., this week to speak privately with him. But once in the meeting, Ms. Brown said, she found him confused or uncertain about why she had come.