Mr. Romney’s appearance underscored the increasingly blurry line between presidential candidates and the so-called Super PACs that have proliferated since a 2010 Supreme Court ruling allowed independent groups to raise unlimited amounts to promote candidates.

Most of this year’s presidential candidates are now backed by one or more dedicated Super PACs. Unlike the broad-based independent groups backing multiple candidates that flooded last year’s Congressional elections with negative advertising — playing a role similar to that of traditional party committees — the new groups are each dedicated to the election of a single candidate.