May 2011, June, July, August, September, October, November, December, January 2012, February, March.
The 2012 Republican presidential primary came to a close in April. The month opened with contests in Wisconsin, D.C., and Maryland. While Mitt Romney was heavily favored to win D.C. and Maryland, Wisconsin was a pro-Santorum state by nature. Earlier in the primary season, Rick Santorum would likely have won Wisconsin with little problem. But by the time April rolled around, Santorum had been weakened by successive defeats and a thorough vetting by the media. Moreover, widely-publicized delegate counts showed Santorum with no chance of getting a majority of the delegates--he could only hope to force a contested convention. Since Republican voters were less than enthused about the prospect of a contested convention, and since they increasingly understood Romney was the inevitable nominee, Santorum's support dwindled.
On April 3rd, the three contests were held, and Romney won them all. Santorum's early lead in Wisconsin was obliterated; Romney won Wisconsin by 7 points. Wisconsin marked a turning point, as no one could any longer deny that Romney was going to win the nomination. Romney was still far from having accumulated a majority of the delegates, but it was only a matter of time.
Looking ahead, April 24th looked to be a good day for Romney: A number of pro-Romney states, plus Santorum's home state of Pennsylvania, were set to hold their primaries. Santorum was faced with a decision. On the one hand, he could stay in the race, attempt to win Pennsylvania, and then ride the pro-Santorum calendar in May. On the other hand, he could save himself the trouble, concede the race, and avoid the possibility of an embarrassing defeat in his home state.
As with Wisconsin, Ohio, and Michigan, the early polls showed Santorum with a shrinking lead. Santorum realized that although he still had some chance of winning Pennsylvania, it wasn't worth the risk. Were he to stay in the race, he would be continually pressured to drop out and make room for the Party's obvious nominee. If he stayed in the race and lost Pennsylvania, it would be a humiliation. On April 10th, Santorum suspended his campaign, conceding the race to Romney.
Santorum's decision effectively marked the end of the primary season. Newt Gingrich attempted to capitalize on Santorum's departure by claiming he was the "last conservative standing." But Gingrich's campaign was long dead, having been killed by Santorum's Southern wins in March. The last gasp of Gingrich's campaign took place in Delaware on April 24th. As with the other states voting that day, Delaware gave Romney the win by a huge margin. Gingrich reportedly made plans to drop out of the race, though he did not make any public statements to that effect.
By the end of the month, Romney was still short of the magic number for a delegate majority, but virtually all opposition to him ceased. Romney was spared the difficulty of dealing with an anti-Romney calendar in May. Romney's campaign and the news media shifted their focus toward the general election.