A Trump-backed pastor is preparing to drop out of Oklahoma's Republican primary after the Daily Mail exposed his intimate relationship with a campaign aide.

Jackson Lahmeyer's campaign manager last night told his team that the married father-of-five was dropping out of the race and taking a sabbatical from preaching, sources told the Daily Mail.

A formal announcement is expected Friday, though some expressed caution that Lahmeyer's pride might still keep him in the race.

Lahemeyer and his wife Kendra invited their congregation to a 'closed meeting' at Sheridan Church in Tulsa at 6.30pm Wednesday.

The Daily Mail has contacted Lahmeyer for comment.