Speaker Paul D. Ryan was briefed a week ago on “credible claims of misconduct” and presented them to Mr. Franks, according to a separate statement from his office. When the congressman did not deny them, the speaker referred the matter to the Ethics Committee and told him to resign. Mr. Ryan’s statement did not detail the behavior in question.
The Ethics Committee released a statement late Thursday saying it had opened an investigation into whether Mr. Franks “engaged in conduct that constitutes sexual harassment and/or retaliation for opposing sexual harassment.”
Mr. Franks, whose strident social conservatism and adamant opposition to abortion in all forms have defined his tenure, said he would step aside at the end of January rather than wait for the outcome of the investigation.
He is the third lawmaker to step down from office this week over accusations related to sexual harassment, as lawmakers in both parties grapple with sexual misconduct in the Capitol.
As it announced an inquiry into Mr. Franks’s behavior, the Ethics Committee disclosed that it had formed an investigative subcommittee to look at whether Representative Blake Farenthold, Republican of Texas, had sexually harassed a former employee and “retaliated against her for complaining of discriminatory conduct.” The committee said it would also examine whether Mr. Farenthold had made inappropriate statements to other staff members.
In an interview earlier on Thursday, Mr. Farenthold balked at the idea of resigning, saying that the news media had treated him unfairly.
“I’ve done nothing wrong,” he said. “I’m happy to visit with anybody who has a concern and explain the facts to the extent that I am allowed to under settlement agreement.”
News of Mr. Franks’s pending announcement swept through the House Republican Conference on Thursday evening as members were on the floor to vote on a stopgap funding measure to keep the government open past Friday. At one point, Mr. Franks and several other Republicans huddled in what appeared to be a group prayer.
Asked earlier in the day — before it emerged that he would resign — about his party’s response to claims of sexual harassment, Mr. Franks had said merely, “It’s a lot to wrap our head around here.”
In his statement later Thursday, he said he was certain that he “would be unable to complete a fair House ethics investigation before distorted and sensationalized versions of this story would put me, my family, my staff and my noble colleagues in the House of Representatives through hyperbolized public excoriation.”
“Rather than allow a sensationalized trial by media” to harm “those things I love most,” he continued, “this morning I notified House leadership that I will be leaving Congress.”
The surprising nighttime development followed a Thursday morning scene on the other side of the Capitol, where Senator Al Franken, Democrat of Minnesota, resigned in the face of mounting accusations that he had groped several women. Representative John Conyers Jr., Democrat of Michigan, announced earlier in the week that he would step down. Mr. Conyers, 88, was the longest-serving member of the House.
Mr. Franks was first elected to the House in 2002. A member of the House Freedom Caucus and a staunch ally of President Trump, he has repeatedly pushed measures that limit access to abortions and other conservative causes.
He represents a district stretching north and west of Phoenix that has been safely Republican. If Mr. Franks indeed leaves office at the end of January, state law requires a special election to fill his seat for the remainder of the term.