How about Strom Thurmond?
No, I have no scenario for him being elected in 1948.
But he was President pro tem of the Senate in 2001, until Jeffords changed parties in June. Assume Gorton defeats Cantwell in Washington in 2000 (Cantwell won by 2,219 votes out of 2.4M). Then it doesn't matter if Jeffords changes parties, so he won't. In any case Thurmond remains as P.p.t. until the end of his term (3 January 2003).
Also assume the 9/11 attacks are postponed until New Years Day 2003, and that the attacks succeed in hitting the Capitol and the White House (instead of a miss and the Pentagon).
Bush, Cheney, and Speaker of the House Hastert are all killed. Thurmond becomes the first (and probably the only) 100-year-old President.