First of all, calling Taft an isolationist a bit of an oversimplification. He was not totally opposed to the Marshall Plan, though he did try to cut its budget. He voted against the North Atlantic Treaty in 1949, but added that he would favor a Monroe Doctrine-like unilateral declaration "to let Russia know that if she attacked western Europe, the United States would be in the war."
http://teachingamericanhistory.org/library/document/speech-on-the-north-atlantic-treaty/ In 1950, he objected to Truman's decision to use force in Korea without consulting Congress, but added that if the question had been presented to Congress, he would have voted to authorize the use of force.
In any event, there is no doubt that he was stereotyped by much of the public as an isolationist in foreign policy and someone who would repeal the entire New Deal in domestic policy. Both of these were oversimplifications, but no doubt they did hurt him, and he certainly would not have won by Ike's landslide margins. But it is by no means impossible that he would win narrowly--Communism, corruption (the "mess in Washington") and Korea (plus a general sense that the Democrats had been in power too long) would work against the Democrats. if he does win, the biggest question is whether he dies on scheduie of cancer, and if so who his vice president would be.