House Speaker John Diehl, arguably the most powerful lawmaker in Missouri, announced Thursday he will resign from office. His announcement came just a day after The Star revealed he’d exchanged sexually suggestive texts with a 19-year-old legislative intern.

Diehl, who will also step down from his legislative seat from St. Louis County, is expected to make the decision official Friday morning.

“I have acknowledged making a serious error in judgment by sending the text messages. It was wrong and I am truly sorry,” Diehl said in a written statement Thursday afternoon. “Too often, we hear leaders say they’re sorry but are unwilling to accept the consequences. … I am willing to face the consequences.”

Hours after Diehl announced he would resign, Republicans gathered to pick Todd Richardson of Poplar Bluff as the next House speaker. Richardson is the majority floor leader. His rise to speaker still requires a House vote, likely Friday.

Diehl had issued a statement Wednesday afternoon admitting the relationship and asking for forgiveness. He told news media gathered outside his office later that night that he had no intention of resigning.

But his survival, or ouster, as speaker was more practically a matter for the overwhelming Republican majority. Those lawmakers met in a party caucus late Wednesday, adjourning with neither a vote of confidence nor a consensus that the 49-year-old should go.

By Thursday morning, the dynamic had changed. Rumors swirled that Diehl’s resignation was a foregone conclusion. The House conducted no business, despite only one day remaining before the legislative session ends for the year.

Many in the statehouse expressed relief.

“It’s time for him to focus on his family,” said Rep. Mike Moon, an Ash Grove Republican and one of two Republican lawmakers to call for Diehl’s removal during the Wednesday night caucus. “Stepping down is the right decision.”

The Star’s story exposed a series of text messages between Diehl and an intern from Missouri Southern State University, as well as the speaker’s various efforts to shoot down the story and delay its publication.

The discovery of the text messages between Diehl and the intern followed a decision by Missouri Southern to end its Capitol internship program last month — more than a month prematurely.

Roughly four hours after Diehl announced his resignation, the intern, Katie Graham, issued a short statement thanking people who had reached out to her.

“Your support means a lot,” the statement said. “This is extremely difficult for both families, and I hope everyone can begin the healing process. I (hope) the Missouri Capitol internship program … remains a positive experience for other students in the future.”

Democrats had immediately called for Diehl’s ouster over the texts. Democratic Gov. Jay Nixon described Diehl’s conduct toward the intern as “clearly inappropriate and troubling.”

The speaker’s actions were an “abuse of power and trust,” said Rep. Lauren Arthur, a Kansas City Democrat, and a sign of the “boy’s club culture in Jefferson City.”

“He is a person who is in a position of power and engaged in an inappropriate relationship with an intern,” she said.

Richardson, the next House speaker, said he will review “our intern policy to make sure we craft the strongest policy we can.”

“That will begin the moment we finish our work” Friday, he told reporters shortly after his Republican colleagues picked him to be their new leader.

Richardson also said the General Assembly needs to gain the public’s trust.

“I don’t think the last five months have put the legislature and this public institution in a particularly good light,” he said. “It’s my great hope that starting tomorrow, we can get back to work and focus on improving that public perception. That’s going to be a priority for me as speaker.”

Some of Diehl’s colleagues who had remained publicly loyal said Thursday that they also supported his choice to step down. Woody Cozad, a lobbyist and former Missouri GOP chairman, said resignation was “obviously the right decision.”

“It avoids dragging out things, makes a nice clean cut and we can move forward,” he said. “I’m sure personally it’s the right thing for him and his family.”

Rod Jetton, a former Missouri House speaker who faced his own set of scandals during and after his time in office, said he was sad for Diehl.

“The young girl was a consenting adult who made a mistake, and John should have been the older, wiser adult,” Jetton said. “He has admitted he made a serious mistake and is suffering the consequences.”

Diehl, an attorney with Husch Blackwell, is married and has three sons.

His legislative legacy will be highlighted by his efforts to successfully override the governor’s veto of a new congressional map in 2011 and a $600 million tax cut in 2014. This year, he shepherded a right-to-work bill through the House and eventually to the governor’s desk for the first time in Missouri history.

He also helped build massive Republican supermajorities in the House, first as majority leader and then upon his election as speaker last year.