With the March 15 slate of Republican primaries in the books, 29 of the 50 states have now voted. Donald Trump, the leader, not only hasn't won half of the votes to date (and hasn't even won half of the votes in a single state), but he hasn't even won three-eighths of them. Rather, Trump has won 37 percent of the vote, Ted Cruz has won 27 percent, and John Kasich has won 13 percent. (Candidates who have since exited the race have combined to win the remaining 23 percent—17 percent for Marco Rubio, 6 percent for the others.)

As of 1:45 EST on March 16, Trump has won 661 delegates (48 percent of the 1,386 allotted to date), Cruz has won 406 (29 percent), and Kasich has won 142 (10 percent). (Others have combined to win 177 (13 percent).

So far, 56 percent of the total delegates available from coast-to-coast have been allocated—1,386 of 2,472. To win a majority of the total delegates—1,237—prior to the convention, Trump must win 53 percent of the remaining delegates. Cruz must win 77 percent of the remaining delegates. Kasich is now mathematically eliminated. Even if he were to win all 1,086 remaining delegates, he would still fall short of 1,237. It is clearly a two-man race.

Some have openly wondered how Cruz can start beating Trump when Trump has won the vast majority of the races so far—and when the Deep South is already in the books. There are three reasons to believe he can do so.

First off, Cruz will benefit from Marco Rubio's exit. For weeks, people have been calling for a one-on-one race between someone and Trump. We're now getting close to that.

Second, we are now entering the stage of the race with closed primaries, those in which only Republicans can vote. To date, the situation has been very different, as 16 of the 20 primaries so far have been open. (The other nine states have had caucuses.) In open primaries, Trump has posted a dominant won-lost record of 14-2, winning 8 of those by double-digits and losing only in Cruz's and Kasich's home states. In closed primaries, however, Trump is only 2-2: He won big in Florida (although still with less than 50 percent of the vote), won pretty narrowly in Louisiana (41 to 38 percent over Cruz), lost to Cruz in Oklahoma (34 to 28 percent), and got routed by Cruz in Idaho (45 to 28 percent). Going forward, 12 of the 18 primaries will be closed, including the biggest ones (California, New York, and Pennsylvania).

Third, the Deep South has always been as much Trump country as Cruz country. Trump does very well with lower-middle-class voters, and all nine Deep South states (counting among them North Carolina, South Carolina, Georgia, Florida, Alabama, Tennessee, Mississippi, Louisiana, and Arkansas) are among the fifteen states nationwide with the lowest median incomes. Trump swept them. Going forward, he'll have to win in more affluent states. So far, his winning percentage is only .500 outside of the Deep South. (He's won 10 of 20 such states.)

The combination of getting closer to a one-on-one race and moving into the closed-primary season gives Cruz a real chance going forward. Furthermore, most of the remaining states are winner-take-all, so he can potentially make up his delegate deficit in a hurry. The flip side is that Trump can get to 1,237 in a hurry if he starts to prevail in winner-take-all states.

Cruz's ultimate prospects for victory may well hinge largely on two things: Will Kasich get out of the race in relatively short order, or will he stay in and serve as an inadvertent blocking back for Trump? And will Cruz strive to widen his appeal a bit within the party and (relatedly) show he's the one who can bring about needed change?

Exit polling in Missouri found that in a Cruz-Trump race, Cruz would have won by a tally of 49 to 42 percent—rather than losing by 0.2 points. Exit polling in North Carolina found that in a Cruz-Trump race, Cruz would have won by a tally of 48 to 44 percent—rather than losing by 3 points. (Even in Illinois, where Cruz lost by 8.5 points, he would have won if the 28.4 percent of the vote that went to Kasich or Rubio had instead split two-to-one for him.) In other words, Kasich's presence definitely cost Cruz a win in Missouri, probably cost him a win in North Carolina, and will surely cost him in states going forward.

(For example, if Kasich sticks around, the key Wisconsin winner-take-all primary in three weeks will almost surely become a repeat of Michigan, where Cruz and Kasich collectively beat Trump by 13 points but individually lost to him by double-digits.)

The only counterargument would be that Kasich might be able to win where Cruz can't. But so far Kasich has won exactly one state—his own. More than one-third of his votes to date—not delegates, but votes—have come from Ohio. He is mathematically eliminated from winning the nomination before the convention and cannot realistically hope to win it there. Cruz has beaten him in 25 of 29 states (with Kasich's wins all having come in New England—where four of six states have already voted—or on his home turf). In short, if Republicans want to stop Trump, the effort would be greatly aided by Kasich's exit.

As for the things Cruz can control, his biggest shortcoming in the exit polling continues to be in the category of which candidate "can bring change." In Missouri, Trump beat him on this count by 10 points (48 to 38 percent). In North Carolina, Trump beat him by 29 points (55 to 26 percent).

If there is any change that a wide swath of Republican voters support, it's the repeal of Obamacare and its replacement with a conservative alternative. Neither Trump nor Kasich is reliable on this front. Trump has praised socialize medicine and has released a health-care plan that would apparently expand Medicaid beyond even Obamacare's levels while keeping one of Obamacare's worst price-spiking mandates. Kasich spearheaded the Obamacare Medicaid expansion in Ohio. Yet so far on this issue Cruz has adopted something of Trump's approach to most issues— I won't tell you what I'll put in Obamacare's place; but, trust me, it'll be great.

If Cruz wants to appeal to a somewhat wider subsection of GOP voters while showing them he'd lead to change, he could release a general-election-ready Obamacare alternative and finally capitalize on what has always looked like his best issue. In the mostly blue, mostly winner-take-all states to come, it might well be the difference between winning the nomination and losing it to Trump.

Jeffrey H. Anderson, author of "An Alternative to Obamacare," is a Hudson Institute senior fellow.