Why would the Republicans support Trump? They would work harder against him
That all depends on your definition of "Republican". Trump has much more in common with those Republicans who are the "base" of the party, who have a majority of Republican voters. The "Republican" East Coast Minority (actually "Democrats Lite"), of course, hates Trump, and, as of now, they control the purse strings and rule-making.
If Trump runs, and especially if he wins, a tide of coat-tail riders acceptable to the "base" will be swept into office, displacing both self-identified Democrats, and the "stealth Democrats", who will have their hot little hands torn away from their current sinecures.
Full disclosure.....I am NOT a Republican. I have been (and am) registered as an "Independent" (and am a strong conservative) since I first signed up to vote (back in the late 1960's)