I happen to think the most terrifying prospect for Republicans would be a challenge to Hillary from the right: the candidacy of Jim Webb, an old-fashioned, truly moderate “Reagan Democrat” with military credentials—Vietnam veteran and former Secretary of the Navy under Reagan—and a lot of appeal in places like Appalachia, and among blue-collar types. That’s a large part of the reason Webb managed to win a term as Senator from Virginia in 2006. (It also happened to be a good year for Democrats.)
For Republicans, Webb could actually be a formidable opponent. Because he’s not far to the left—he endorsed Republican Senator George Allen in 2000, before running against him six years later—he wouldn’t mobilize the conservative base in opposition the same way Hillary Clinton or someone farther to the left would. He would have crossover appeal; an old Reagan Democrat could win the votes of other old Reagan Democrats who have gradually become Republicans or right-leaning independents. And he would probably still get the votes of Democrats, who would be mobilized in opposition to the Republican candidate, who will either be a scary conservative or someone named “Bush,” either one of which is anathema to them.
All of this would be a big problem, if Webb were the nominee. But I feel safe giving this free advice to the Democrats precisely because Webb is an old-fashioned Reagan Democrat, so he won’t be the nominee.