For Frank Church to win, he needs to make a very difficult choice. In OTL he chose to remain in the Senate to continue his investigations of the Ford Administration and the Intelligence Community. This was his major focus, and the major goal of his career. He entered the race late in an attempt to stop Carter, who he rightfully saw as unable to lead the nation in the right direction.
But if Church decides to run from the very start, he has a shot. More than any other candidates (besides Carter) he's the most likely to embrace the McGovern reforms and actually campaign in Iowa and the other early states. He's also the ideal candidate in Iowa, considering he is from a farm state, is pro-gun, pro-farmer, and a foreign policy dove. If he wins Iowa, he could go on to win the nomination in a similar manner to Carter. At that point he beats Ford, possibly by a wider margin. His running-mate is probably Lawton Chiles, Fritz Hollings, or John Glenn. Someone to give regional balance.