Ike was popular but the GOP was not.
This was the height of the country club GOP era. The Republicans had not broken into the suburbs of the South as much yet. They were basically a Northern Suburbs plus Vermont (insert ancestrally Republican rural areas) Party. Rockefeller and Javits types could win upscale liberals in urban areas as well as a lot of working class types in those areas. However, union dominated and ethnic urban and surburban areas were Democratic Territory in the New Deal Era. In order to win a majority these people had to vote Republican or not turn out (think back to 1920 when they didn't turnout and the Republicans won supermajorities because of it. By the 1950's they needed that same effect, just to win a majority in Congress).
There was the recession of 1957-1958, which moved many of those voters to turn out for the Democrats.
^^ This. And Ike was very popular among Democrats, too, because of his moderate positions. He wasn't a true Republican. He decided to run for the GOP because the country had 20 years of Democratic presidents in 1952.
