Mr. Weaver, 61, is a longtime Republican presidential marketing campaign adviser who gained prominence throughout John McCain’s run in 2008 and likewise labored for John Kasich in 2016. The Instances reported last month, based mostly on interviews with 21 younger males, that Mr. Weaver had for years despatched unsolicited and sexually provocative messages on-line. The youngest individual The Instances interviewed was 14 when Mr. Weaver first contacted him; the messages grew to become overtly sexual after he turned 18.

On Thursday, The Associated Press and New York magazine, citing unidentified former staff, reported that Lincoln Mission leaders knew about Mr. Weaver’s conduct final summer season, which Mr. Schmidt has continued to disclaim. Mr. Weaver took a medical depart from the group in August and introduced final month that he wouldn’t return.

In its statement on Thursday evening, the Lincoln Mission mentioned that Mr. Weaver had “betrayed all of us” and referred to as accounts of his harassment of younger males “heartbreaking.” It mentioned that Mr. Weaver’s conduct “should be reckoned with,” and mentioned it was bringing in “a best-in-class outdoors skilled” to “set up each accountability and finest practices going ahead for The Lincoln Mission.”

The previous staff, of their letter, expressed anger that Lincoln Mission leaders had characterised stories concerning the group’s dealing with of Mr. Weaver’s conduct as hit jobs from supporters of former President Donald J. Trump.

Insinuating that their efforts constituted a right-wing assault, they wrote, “shouldn’t be consistent with the values we signed as much as uphold, and resembles the ways and conduct we joined the Lincoln Mission to defeat.”

Over the past yr, the Lincoln Mission established itself because the main Republican group against Mr. Trump, skewering him with mocking advertisements and drawing a big following on the left. However for the reason that election, the group has been splintering. Two former board members, Ron Steslow and Mike Madrid, left in December. George T. Conway III, one other key determine within the group, has additionally departed.

Ms. Horn mentioned in her statement, “When The New York Instances report on Weaver got here out just lately, I began getting cellphone calls from some victims who shared very disturbing tales about their interactions with him — interactions that apparently began almost a yr in the past and, in keeping with these younger males, have been communicated to others within the Lincoln Mission.”