(Updated 4:45 p.m. ET)

Despite his capacity for self-sabotage and the public shunning of his own party, some well-heeled Republicans continue to believe that it's worth investing in former South Carolina Gov. Mark Sanford's comeback bid in Tuesday's special election for a vacant congressional seat in the Palmetto State.

During the last three weeks of the campaign, the unexpectedly competitive contest in a heavily GOP district between Sanford and Democrat Elizabeth Colbert Busch has become a magnet for big political spenders. Sanford, attempting a redemption run after a headline-making sex scandal that cost him his ...