IOTL, New Jersey Senator and former New York Knicks star Bill Bradley was the lone challenger to then-incumbent VP Al Gore in the 2000 Democratic Primary. Running to the left of Gore, he didn't win a single primary, and took only 21% of the vote.
Your challenge, as the tin suggests, is to both change the outcome of the primary that year and then have Bradley go on to win the Presidential election. A longshot, I know, but i'm curious to see if anyone can think of a way to achieve it.