If Trump can be persuaded to step down (and now is the perfect moment to apply the pressure), then Pence should be the new nominee. I don't know what the rules are (if in fact there are rules for such a contingency), but Pence won't split the GOP the way Cruz or Kasich would. That's because Pence has the imprimatur of Trump himself, and won't cause the Trump supporters to bolt. Pence has the best shot of uniting the three wings of the current GOP - the Trumpsters, the NeverTrumps, and the moral wafflers that Bret Stephens refers to as the go-along Republicans.
The time is now, folks - Trump's feud with a Gold Star family renders him toxic, and his refusal to come clean about the Russian connections to his businesses creates an unacceptable conflict of interest. A united push by prominent Republicans to get Trump to step down could not only succeed, but create a united GOP in its midst.
It's time to choose, folks, for the future of conservatism and the nation.