Rocky (and I don't mean Conor Flynn 
), had the potential but never followed through. I wouldn't called it disappointing because he didn't even win the nomination.
IMO, it has to be Al Gore. He had that election on a silver platter. The 2000 election for the Democrats should have been like 1988 for the Republicans and they blew it big time.
The dynamics of 1998 screwed Gore. In 1997, Gore was leading Bush in the one or two polls which came out. In 1998, Bush led Gore in every poll, often substantially, and kept this lead until September of 2000.
The Monica Lewinsky scandal came out in early 1998; this disgusted the electorate, and Gore was (unfairly in my opinion) tarred with this.
The Republicans in the House however, started attacking Clinton hard and this created a backlash. People didn't like seeing their President attacked like that when things seemed to be going well. So they rewarded Clinton with high approval ratings and blunted the GOP in the 1998 Congressional elections.
But their digust at Clinton's perjury and infidelity had to be displaced somewhere, and it was displaced upon Gore, who represented 'the administration';
In contrast was Bush, who represented 'traditional values' and everything that the Lewinsky scandal offended.
At the time, the perception was that the GOP had miscalculated by going too far. People didn't notice the persistent Bush lead or didn't connect it to the Lewinsky scandal. The 1998 Republican failure had a healthy, moderating impact on the party and the religious right would remain quiet as their candidate, George Bush, ran on a platform of 'compassionate conservatism'. So in a way, the Lewinsky scandal did hand the GOP the White House.
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George Romney was also hurt when Lyndon Johnson said that his view on Vietnam was precisely the same as Romney's. It made Romney seem colorless. Romney also failed to court the emerging southern wing of Republican kingmakers.
