The difference between falling short of the threshold in these seven states and clearing it, even if only barely, is approximately 70 delegates. It’s not enough to preclude a candidate like Mr. Rubio from winning the nomination. But it would require him to fare very well from that point on to finish with a majority of delegates; he might need as much as 70 percent of the outstanding delegates to win, a plausible figure given the party’s delegate rules but nonetheless a daunting one.

The delegate threshold challenge poses big questions for campaigns, which will have to choose between strategies that maximize delegates and those that maximize momentum. That’s because many of the conservative, Southern states where the mainstream candidates need to clear delegate thresholds on Super Tuesday aren’t necessarily the same states where they have the best chance to win.

Texas, Alabama, Tennessee and Georgia are all fairly conservative states where Mr. Trump or Mr. Cruz will be favored. Texas is also Mr. Cruz’s home state. The mainstream candidates might have a better shot to win in states like Virginia, Minnesota, Massachusetts or Colorado, where the delegate thresholds are lower (10 percent or less) or nonexistent.

The mainstream candidates are working out this calculus: Is it worth it for a campaign to spend millions for third place and 21 percent of the vote in Texas, which would yield many more delegates than third place and 19 percent? Or is it better to spend the money to win states like Massachusetts or Virginia, where the candidates are all but assured to win a modest number of delegates and won’t necessarily earn many more delegates by spending big to win?