But here in the state with the first nominating contest, about six weeks away, Mr. Trump has put off the nuts and bolts of organizing. A loss in Iowa for Mr. Trump, where he has devoted the most resources of his campaign, could imperil his leads in the next two nominating states, New Hampshire and South Carolina, where his get-out-the-vote organizations are even less robust…

Mr. Trump’s Iowa director predicted that he would recruit a leader for each of the state’s 1,681 Republican precincts by Thanksgiving. Instead, the first major training session for precinct leaders, heavily promoted in emails and conference calls, drew only about 80 people to West Des Moines last weekend, with about 50 participating online.

Some of Mr. Trump’s Republican rivals have spent months calling and knocking on doors to identify potential supporters to draw them out to caucuses, but Mr. Trump does not appear to have invested in this crucial “voter ID” strategy until recently…

Interviews in Iowa with Mr. Trump’s campaign workers, his volunteers and dozens of attendees at his rallies over two months, as well as observations of voter outreach, conference calls and confidential training sessions, indicate that Mr. Trump’s support in the Feb. 1 Iowa caucuses may fall short of his poll numbers in the state. He is now trading the lead position with Senator Ted Cruz of Texas.