Well, a tiny fraction less than half the American people, at least the first time?

All those who voted for him and all those who didn't vote against him are equally complicit

In 2016 Trump got 46.1% of the popular vote vs Hillary Clinton's 48.2%.

Expressed as fractions of the total US population in 2016:

  - about 28.5% of the population was not eligible to vote (due to age or other reasons)
  - about 31.6% of the population was eligible to vote but did not vote for either candidate
  - about 20.4% of the population voted for Hillary Clinton
  - about 19.5% of the population voted for Trump