For Bush the elder it was the success of Operation Desert Storm and of course the infamous broken "READ MY LIPS, NO NEW TAXES" pledge that successfully ended his bid for another term.
For Clinton, it's the conquest of Monica Lewinsky "I DID NOT HAVE SEX WITH THAT WOMAN" singlehandedly redefining the sexual mores of oral sex as not really sex.
And now we have Dubya who's defining issue will of course be 9-11 and his response to it. The WOT is now the most important issue of our time, it's all about our survival as a nation and anything related to it can mean success or failure to Bush's legacy, luckily this Amnesty Bill did not pass so there's still hope that Bush can leave office on a good note rather than a disastrous one.
“SEE YOU AT THE SIGNING” may go up there with “READ MY LIPS”