Interesting. I thought he based it off Nazism what with the ever present use of red and black as the empire's colours (plus the white from the storm-troopers)...I think even the empire's flag was supposed to be the black imperial emblem in on red field with white inside the emblem.
Of course, no reason Lucas wouldn't mix and match - So Nixon + Nazism = Evil Empire.
Lucas based the Empire on Nazi Germany to a large degree (and the villains of samurai movies), but Nixon got the concept ball rolling.
Just imagine the Emperor speaking like Nixon.....
"And people have to know, is their Emperor a crook? Well I'm not a crook. I've earned the right to kill every Jedi I have."
As for the OP, I think this has to one of the harder challenges. Hard to see it getting repealed, even if Nixon has mass popularity and no Watergate. Much more likely as had been said is just to get it modified (via another amendment of course) which only limits the 22nd amendment to just two consecutive terms. As it is now and then, the amendment basically made sure that the other party got a shot at the presidency by ensuring that any uber-popular incumbent doesn't pull a Roosevelt and keep the other party out of presidential office for over a decade. However having a limit be 2 consecutive terms would allow for fresh candidates (and thus a chance for the other party to get it's candidate into the white house) while removing the lame-duck syndrome as the incumbent in his (or later "her") second term could still run for election after 4 years and so they wouldn't necessarily become a lame duck by the time of the next election campaign.
I think it's reasonable enough, especially for Nixon. Republicans supported him; Democrats (to a large degree supported him); and almost everyone else supported him. Nixon also carried that fear of trying something different syndrome. America had gone through upheaval, and Nixon had dealt with that, and was America's safety blanket.
It's pretty hard to imagine even the most "bipartisan" politician supporting a constitutional amendment designed to ensure the re-election of a member of the opposing party. The 22nd Amendment is "sticky" in that way -- there's a built-in constituency that's pretty much always going to be large enough to block its repeal.
The parties weren't of solid ideological blocks. There isn't one Conservative party, and one Liberal party as there is today. The Democrats had a Liberal, Moderate and Conservative faction as did the GOP. And Nixon played to each faction. Western Conservatives/Libertarians mistook his anti-communist language for anti-government language, he had middle America locked up from his pro-Vietnam and patriotic language, and his domestic policies rubbed Liberals a good way, as well as minorities. So partisanship really relied on party name a lot of the time. It's basically like cheering for the Packers against the Bears. Ain't too much of a difference except superficialities and different people, but a loyalty remains nonetheless, and that's the major thing that could derail an attempt for a repeal.
But, as said, Nixon had popular support among Americans, popular support among a swath of politicians (many, many Democrats included), and it would likely be a heated debate.