In primaries, there’s a well-known bandwagon effect — voters like to cast their ballots for candidates who are doing well — and in the last few weeks that has benefited Cruz. But if Cruz fares poorly in upcoming Northeastern and mid-Atlantic states (New York, Pennsylvania, Maryland, Delaware, Connecticut, Rhode Island), he could lose some of those newly gained Rubio voters to Kasich. While the Rubio voters are close to Cruz ideologically, their income and education levels make them closer demographically to Kasich voters. Kasich likely wouldn’t take all of Rubio’s voters from Cruz, but if he takes too many, he could potentially split the non-Trump vote and help Trump take upcoming winner-take-all and winner-take-most states.
Additionally, if potential delegates see Cruz losing steam, they may be hesitant to sign on for his delegate selection effort. These potential delegates will feel a great deal of pressure not to subvert what they see as the democratic will of the voters. So if Cruz is doing poorly at the time they’re selected, they might try to keep their options open rather than committing to Cruz’s stop-Trump effort. While many of these delegates likely won’t favor Trump — the party regulars who usually participate in this process mostly don’t — Cruz would probably prefer that they commit to him rather than keep their powder dry.
Note that none of this is a prediction. Cruz might outperform expectations in the upcoming contests and continue to build on his recent successes. But this is all to say that there’s uncertainty surrounding if and when the good news will stop rolling in for the Texan.