If the front-runners collapse, many viable candidates will bounce into the race. However I’m unsure there’s anybody I’d describe as “stepping in,” which at the least to me implies somebody sturdy, ready within the wings, and able to take over and restore order to the state of affairs, form of like if Joe Biden had stepped in if Hillary Clinton had been sidelined for any cause in 2016. This time, Ted Cruz may be the closest analogue. Maybe Mike Pence may nonetheless play an identical position, too.