Truman actually thought Taft would be the better candidate in 1948, so I presume he would run even harder in 1952. And he would enjoy it a lot more. Ultimately, Truman would eke out a win because Eisenhower would not back Taft. But Truman was really unpopular, it wouldn't be a landslide. The sheer lack of a Republican landslide means the class of '46 is cut down a bit earlier instead of decimated in '58. The really interesting thing is how Harry Truman works with Majority Leader Lyndon Johnson. More of the Fair Deal will get through with LBJ's backing.
If the Democrats do better in the Senate elections, then Ernest McFarland almost certainly wins relection in Arizona, which means he'll continue as Majority Leader. At any rate the Fair Deal will be dead in the water no matter who the Majority Leader is. The conservative coalition in Congress is just too strong at this time for any major legislation to get passed. (In fact you've probably got a better chance of seeing federal aid to education and increased public housing and hospital construction enacted under President Taft than under Truman.)
Truman would win it. The average American was a lot more liberal than. When Taft opens his mouth he was doomed. Plus Ike would not support Taft. Good question who might be the VEEP candidates? I don't think Truman keeps his. Who would Taft pick? Also maybe Truman puts Ike in Korea, has a General. Or part of the cabinet.
Taft was able to win three statewide elections in Ohio, so he obviously could appeal to more than just conservative voters. And Taft had rather liberal positions on public education, public housing, and hospital aid. Nor were his conservative positions on taxes, spending, and unions unpopular in the country at large. (Even large numbers of Democrats supported Taft-Hartley.)
And Taft's veep would probably have been William Knowland. Young senator with war experience from an important state, who is conservative enough to satisfy Taft, but has connections to the moderate wing of the party (Knowland was a close political ally of Earl Warren.)