Mike Huckabee, judging by the early opinion polls, would have been among the most formidable challengers to Mitt Romney, the putative front-runner for the 2012 GOP presidential nomination. But Huckabee's departure from the race, paradoxically, could make it more difficult for Romney to prevail. The reason is that with Huckabee off the field, the former Baptist minister's core constituency -- the evangelical Christians who represent nearly half of the GOP's primary electorate -- are now back in play for all competitors. If Romney can't defang the resistance he encountered from those voters in 2008, he faces the threat that they...