There’s a clear and impermeable line between the progressive mainstream and the left fringe. The line between the Republican Party/conservative movement and the far-right fringe barely exists.
Whereas Dems kept the fringe at arm’s length, Republicans embrace the fringe with both arms. Both sides have nutjobs; only one side thinks their nutjobs are sane.
An example, just from today. When Barack Obama campaigned last year on increasing troop levels in Afghanistan, he got very little support from Republicans; nor had they complained about inadequate troop levels there during the Bush years. When Obama took office, there were fewer than 32,000 troops in Afghanistan. There are now more than 60,000. The President has just announced a decision to take that level to 100,000. And Republicans are complaining that the proposed level isn’t quite as high as the generals in theater asked for. (Worse, the press is reporting those complaints straight, rather than openly mocking them.) That sort of deficiency in basic reality-checking is not characteristic of a party ready to resume power.