In a close election, he'll get about what Romney got or a little less, losing support among traditionally Republican college educated men, especially those in suburban areas (especially in the North) and gaining support among some high school educated and lower income White men in the Rust Belt and Appalachia who usually voted Democrat.
How is that plausible? If the election is close, he'll most likely do notably worse than among women and Hispanics (even if slightly better among blacks).
In a close election, better than Romney. Trump doesn't have much of a chance of winning if he doesn't get more than 2/3 of white men.