Adlai Stevenson's third run in 1960 was tossed aside despite the Democrats having stronger chances than both of Stevenon's nominations. Richard Nixon waited eight years to stage his comeback. Hubert Humphrey was tossed aside in 1972 after losing a close election to Richard Nixon. Likewise, Ford and his running mate Dole nearly beat Carter four years later, but was only given mild speculation for a second run in 1980, with his former running mate falling behind not only frontrunner Ronald Reagan but CIA Director George H. W. Bush in polls.
Those are actually all bad examples.
1. Had Adlai actually tried to take the nomination a third time, he probably would've have gotten it. The sheer enthusiasm displayed for him at the convention OTL scared the hell out of Kennedy.
2. Nixon tried to maneuver himself into become the compromise candidate of a brokered convention in 1964.
3. Humphrey nearly won the nomination in 1972, but McGovern knew the ways of the new primary better (because he had reformed them himself), and narrowly got the nomination.
4. Ford didn't run in 1980, but Anderson and Bush split the left-of-Reagan vote. Had that vote coalesced around a Ford candidacy, a nomination is possible.
And this is all ignoring the fact that Bush had Lee Atwater, a campaign weapon of mass destruction who would exploit any shred of scandal he could find on the bottom of Clinton's shoe. If not for Lee Atwater, then we wouldn't the infamous 'Willie Horton' ad, and thus much less emphasis on crime, hopefully butterflying away the hypothetical rape of Kitty Dukakis being discussed on national television as a debate question - and then without those two embarrassments, Michael Dukakis might actually have gone on to be President of the United States.
Whoa, whoa whoa. Clinton isn't Dukakis. Clinton went out of his way to execute a mentally ill black man just to prove himself immune to the kind of things Atwater would throw his way.
I don't see how he wins the nomination in '88 though, with him having to compete with Gore on his right and Jackson on his left in his native South.