It's easier to just have different people with with a few of them.
Or, in the case of FDR, have WW II end early. Who runs in '44 for the Democrats if, say, the Weimar Republic survives, and the japanese attack us and are defeated early? Or, would he even run in '40 without hte threat of war; Europe was a bigger threat at the time he decided to run, IIRC. Robert Taft?
Absent the threat of war in 1940, the contest for the GOP nomination was largely between Taft and Thomas Dewey, with Senator Arthur Vandenberg of Michigan as a sort-of-close third contender: at least, that's the way it shaped up until just weeks before the convention when Willkie came out of nowhere. Had there been no threat of war, I'd guess Taft would have gotten the nomination. His running mate would be more problematic: do the Republicans look for a balanced ticket, and thus choose someone from the northeast (say, Leverett Saltonstall of Massachusetts) for the second slot, or do they go for ideological purity and take someone even more "orthodox" than Taft (say, Karl Mundt of South Dakota, or Thorkelson [don't recall the first name] of Montana)?
But go back forty years more and say McKinley is only wounded (Czolgosz sneezed at the critical instant, throwing his aim off) and serves a full second term. Now TR is only another vice-president (although admittedly a very dynamic one) who will try to secure the nomination for himself, but it'll be an uphill battle: he'll have to go against the moneyed conservative wing of the party, which is likely to want someone along the lines of Philander Knox or Charles Fairbanks as the nominee. Besides, no sitting vice-president has gotten the nomination since Van Buren. And since the Democrats have nominated the conservative Alton Parker, it looks like it's safe to nominate someone friendly to Wall Street. As such, the GOP puts forth a ticket of Knox and Fairbanks in 1904, resulting in a relatively close GOP win by plurality (there's enough revulsion in the working classes to give Eugene Debs significant support).
Now you've set the stage for another four years of a conservative, relatively weak presidency--at a minimum. And even if the Panic of 1907 ensues as it does in OTL, the Democrats will likely nominate Bryan again to run against Knox and Fairbanks, but Bryan is not exactly the model of a powerful president, either: he'll preach, but not act. Power will remain consolidated on Capitol Hill, rather than at 1600 Pennsylvania Avenue.
Fortunately, TR is still young--only 50 in 1908--so that he may well have the chance to run again after, say, another term as governor of New York. But by then, a lot of damage has been done, and it'll take a lot of doing to break the US loose from its insular model and transform it into a real world power (from its status as a regional power at best).