Here's why, at least temporarily, the Democrats are winning and the Republicans really can do nothing except hope for failure and try to hijack the process:
For at least three decades, the Republicans have hitched their wagons to an ideological and economic model that has proven to be a failure.
The Democrats have done a better job hiding their connections and dependence on said economic model. Some of them are even being coaxed into a path away from that model, however slightly and uncertainly (and with right wing cries of "socialism" every step of the way, a ritual that has now become comical and fails to convince a public that genuinely doesn't believe a more "socialistic" --regulated and egalitarian-- model would be any worse than what the neoliberals and free marketers have produced, and they're right).
There's at least some hope and mystery in the Democrats. The Republicans literally, and I mean literally, still believe in Reaganomics, trickle down, and deregulation. They tell this to the public and get about 25% of us to agree with them. They may as well be trying to pitch Molly Hatchet albums, pet rocks, or NAMBLA memberships. Keep up the good work, boys.