Let us take a look at the electorate of this country. Al Gore nearly beat George W. Bush. Then John Kerry, a super radical and proven traitor to the country, came within two states of beating Bush. Then Barack Obama, another super-radical with zero accomplishments, beat war hero and accomplished senator John McCain, and then again easily defeated boy scout Mitt Romney. Given all that, is there any question that this same electorate would not embrace a radical California lefty over Donald Trump?
Those of you who think the problem is that we are not nominating conservatives are deluding yourselves. If a conservative cannot win a GOP primary, what makes you think a conservative can win a general election? The parties are realigning for a reason. There is no possibility for conservatism alone to get a sufficient plurality of the vote in this country.
I guarantee you that if Trump loses this election, the GOP will not "go back to its conservative roots." It will instead build on its nationalist base. Trump dominating the party is a strong indication that the nationalists outnumber the conservatives at this point.
The key tenet of nationalism (which I disagree with) is that we need to strengthen our borders and restrict trade in order to protect our social safety net. This was basically the position of mid-20th century Democrats like FDR. That position is in direct opposition to traditional conservatism, which focuses on smaller government, individual rights, and the preservation of traditional institutions.
Given that the left has now embraced global tyranny, I don't see any other option available to conservatives except to join forces with the nationalists to defeat the left, because global tyranny is bad. Very bad.