Hillary Clinton beat Donald, in the popular vote. So, I would say any accepted Democratic candidate can beat him because of the loyal Democratic bloc who would never vote for Donald.
Even so, of course, the electoral college voted him in, but now there is indication that the Russians could have helped tip the scales so Donald got as many votes as he did. If the Russians had not done that, may be Hillary would have won.
And so I can see it possible that without Russian interference any Democratic candidate could beat Donald, just because of Democratic voters' loyalty to their party.
He has not gained any more favor of the loyal voting Democratic voters, possibly. So, that bloc of votes could stay the same. But without Russian help Donald might not get so many votes. I have not found any official indication about how many votes could have been swayed away from Hillary and to Donald, because of the Russian interference.
We will see.
Then there is the matter of how "God resists the proud" (in James 4:6, 1 Peter 5:5). How He resists the proud can decide who gets in. But our humble and caring prayer for any and all people will have His result. So, it is wise to love and care for and pray for every candidate > "first of all" > 1 Timothy 2:1-4.