Reuters-Ipsos recently added a daily tracking poll querying participants on who they'd pick in a three-way race between Trump, Cruz, and Rubio. Among Republicans, the results shake out like this:
And among independents/third-party members:
As a Trumpian, I was initially apprehensive about too much of the field dropping out after the first few states had voted, concerned that most of the capitulator support would go from one dead GOPe-approved candidate to another one still standing. A brokered convention would potentially allow for a corrupt bargain to be made if Trump had won something like 40% of the primary electorate to second place's 25%, but much as it would pain the party leadership to do it, I suspect they'd still give Trump the nomination in that scenario in exchange for some concessions and the VP spot.
To withhold the nomination from Trump in a situation where he had an indisputable plurality of the voters would invoke the Samson option and virtually guarantee a Democrat blowout in November. Sure, a lot of the GOPe would prefer Hillary to Trump, but most voters--even those who voted against Trump in the primaries--would be angry about the general election being conceded like that, and only the cuckiest of cuckservatives would feel Trump unjustified in refusing to endorse the non-Trumpian nominee in such a scenario.
Narrow the field, though, and the potential fact that Trump's Republican ceiling was below 50% could be revealed.
It looks like I was too pessimistic, however. If just 1-in-5 of either Rubio's or Cruz's supporters went to Trump or to "wouldn't vote", Trump comes out ahead in a head-to-head race, and this is without taking so-called electoral momentum into account, something that is probably especially beneficial for a 'controversial' candidate like Trump, for whom some people are waiting for a social signal that it is indeed okay to vote for him.
If it's Cruz that bows out first, it'd be shocking if less than 20% went to Trump. I suspect it'd be closer to 50%-50%. Rubio calling it quits would be a little more precarious, but Trump would still probably get there and anyway that seems unlikely given how the GOPe would really have to hold its nose to put pressure on Rubio to drop out so it could throw all its weight behind Cruz.
Parenthetically, comparing the daily tracking poll that still includes Kasich, Carson, and ¡Jabe! with the three-way poll suggests that if the second-tier were to collectively disappear, Rubio would get 50% of their supporters, Cruz would get 30%, and Trump would get 20%. ¡Jabe! and Kasich will surely benefit Rubio, while Carson could be better for Trump and Cruz.
And if this analysis isn't convincing enough, consider that the markets appear to be coming to a similar conclusion. Trump is now given a better than 60% chance to win the Republican nomination (and that prices in any sort of convention shenanigans that might keep it from him), Rubio just over 30%, and Cruz less than 10%.