If the 1948 US Presidential Election hasn't taken place yet, I assume it'd be Truman going up for re-election against some Republican candidate- either Eisenhower (SHAEF general, nationalist), Dewey (domestic policy, IRL nominee), or Taft (conservative, anti-New Deal). It's a tangled mess when considering the politics of our universe's America, but if you ask me, it's possible Eisenhower comes close (assuming he doesn't leave the Army) or maybe Taft, in a neck-and-neck with Truman.