@Dante is probably being a bit over-dramatic and over the top in assuming that Hilary will lose in 2020 but he makes a fair point: very few parties remain in government for longer than 12 years, and Hilary's been up against Trump.
Yes, Trump did beat the rest of the Republicans in the primaries, but I think his presidential campaign has been demolished by his own failings: namely his incoherence, lack of genuine policy, sexual assault allegations, and splits from the Republican party. His campaign has been ridiculously, absurdly shoddy, full of basis errors, and a lot of his ads have been very amateur in comparison to what Clinton's produced. If
that can come close to a President, then a well-run, scandal-free campaign from a Republican will be a massive, massive threat. Someone who can point out Hilary's many flaws, without having too many of their own that deters voters. Granted, there's a lot I'd take issues with in
any Republican...but it's the general public they have to appeal to.
Certainly, when you consider the establishment nature of the GOP, the Dems may find themselves in a tough spot when
they are the ones being perceived as the establishment, and the Republicans are being seen the genuine party of change. Someone who can capture the more conservative, religious aspect of the Republicans
and mix it with the anti-establishment rhetoric of Trump (even in a hypocritical manner) has a good shot. Whether that candidate exists is another question.