Rolf, Here's the problem. Not all republicans... even most of them are white nationalists, racists, bigots, anti Semites or in clinton's word "deplorables"... but all of these folks, who call themselves members of these groups are aligned with Trump. It took Trump a week to decline the support of one of the countries high profile politicians, an ex KKK leader. These kind words were not even as subtle as dog whistles. The behavior has caused many prominent members of the party to renounce ever voting for him... much less supporting his agenda, or fitness for office. In a two party system.... they are forced to choose.... and in not choosing risk the fates of what they see as a dangerous outcome and then be forced to shoulder their responsibility or lack there of.

These never trumpers consider that allowing ANY of these "deplorables" a seat at the table or even in the room as unacceptable and counter to their conservative governing agenda and their ability to get 270 electoral votes. 50 years ago, the conservative movement threw out the John Birch Society and the Koch brothers father for their antisemitism. .... they managed to elect Reagan and two Bushes holding to those principles by keeping the deplorable far far away ... Now even the Koch brothers decline to support Trump because the company he keeps is unacceptable.

So, the old saying.... sleep with dogs... don't be surprised by waking up with fleas is advice they are taking to heart.

Perhaps in your parliamentary system, the option of a voting for a minority party and hoping for a coalition government is a choice... but not in the States.