He can't. This is a planned, well orchestrated strategy on the left and it won't stop unless Cain's popularity rose after each attack. If Gingrich keeps rising in the polls, the same will happen to him - serial adulterer, served his wife on her death bed, etc. After these two, anyone else who might be a threat (except Romney) will get the smear treatment; because it works. Romney will stay clean until the general election if he is chosen. Then we will get endless articles about the evils of Mormonism and Mormons' bloody past, etc. This is modern politics in America.
As a matter of fact, I've read that Hollywood is planning on releasing a movie based on the book Under the Banner of Heaven (which is a damning indictment of fundamentalist Mormon cults) next summer,just in time for the elections.Just a coincidence, I'm sure/s