Probably Democratic unless a third party can put up someone credible. If I don't live in a safe state by 2020, I'll definitely vote for the Dem candidate.
What would a "credible" third party candidate look like to you? Someone who can reach Ross Perot 1992 levels? Because I don't see any third party getting above that under the current political system.
Even someone at the level of John Anderson in 1980 could take enough conservative votes away from Trump to ensure a landslide for a Democrat who gets a bare majority of the vote. Obama got a slightly larger percentage of the vote in 2008 than did Reagan in 1980.