A few days ago, we reviewed the situation in Wisconsin, which holds its primary on Tuesday. If the polling is to be believed, Romney will carry Wisconsin and win the other two contests that day. Afterward, Elephant Watcher expects the nature of the race to change--such that it's not viewed as a real race anymore--with virtually all observers recognizing that Romney will win the nomination. After Tuesday's contests, it will be another three weeks until the next series of states will vote. It will not be a pleasant three weeks. Will Santorum quit?
The problem with inducing Santorum to quit after April 3rd is that Santorum's home state, Pennsylvania, will be among the states voting in the next batch (April 24th). The other states that day will vote for Romney. While Pennsylvania may be more competitive than expected, it should favor Santorum. How can Santorum quit right before his home state votes? If polling shows Santorum a certain loser in Pennsylvania, then he may be inclined to quit. But that's unlikely. If Santorum stays in and loses Pennsylvania, he'll have a good reason to quit.
After April 24th, Romney's lead in delegates will be at its greatest. The delegate math will be unquestionable, even if Santorum does well in Pennsylvania. This would be a good time for Santorum to end the race. Unfortunately, the calendar will again tempt Santorum to keep fighting. In May, the calendar is extremely favorable to Santorum, with many Southern states voting. The big one, Texas, will not vote until the end of May. If May's roster of states had been earlier in the calendar, Santorum would have some real momentum. (The newsmedia, which tends to overreact to short-term trends, could even fool itself into believing Santorum is getting somewhere in spite of the math.) By the time May rolls around, the math will not permit Santorum to win. However, the allure of winning all of those easy states will be very enticing to Santorum.
If Santorum is unable to resist staying in the race, he should do well in May. Nevertheless, the effect of Romney's wins in April will have a profound effect, and could lead to a collapse in Santorum's support. If Santorum manages to lose Texas, then he should quit, because the states voting in June will guarantee another string of humiliating losses. But if Santorum wins Texas, how can he immediately quit? One doesn't normally quit after a win; candidates quit after losses. So if Santorum stays in the race after Texas, he will essentially be staying in until Romney gets a majority of the delegates, which will occur on June 5th.
In summary, the delegate math and establishment pressure will suggest Santorum should quit, but the primary calendar will repeatedly encourage Santorum to stay in the race until Romney has won a majority of the delegates. Only a loss in Pennsylvania (or Texas) would be a likely trigger for an early exit by Santorum.