After a week of intense campaign activity that was supposed to clarify the Republican presidential field, the clamor for new entrants has intensified, with fresh hints from Sarah Palin that she could jump in and pressure mounting on New Jersey Gov. Chris Christie and Wisconsin Rep. Paul Ryan to join the race. The activity suggests the extent to which dissatisfaction with the current field lingers—as well as the possibility that President Barack Obama's growing problems could entice others to seek the chance to unseat him. "Do I think the field is complete yet? I would have thought so, but I've...