March 15 is a key date on the Republican primary calendar. Contests held on or after this date may be "winner take all". The candidate winning the most votes, even without an outright majority, can win 100% of a State's delegates. Party rules were designed this way to help a front runner pull away and collect an outright majority of delegates. With Trump as the front runner this year, many establishment Republicans are regretting this rule. The pressure is now increased to make this contest a two person race, which is the only way I see of anyone defeating Trump.
The two most likely drop out candidates are Marco Rubio and John Kasich. Between them. they have only won one State (Rubio in Minnesota). Rubio has also won contests in Puerto Rico and DC. Kasich has won nothing yet, a poor precedent for moving into winner take all States. Both candidates have said or strongly implied that failure to win their winner take all home state (Kasich in Ohio, Rubio in Florida) will end their campaigns. Trump is hoping to defeat both (and is ahead in most polls in both States) and knock out his few remaining impediments to the nomination. Cruz also seems to be battling for votes in these two States. Support for Cruz makes it harder for Kasich and Rubio to win. Cruz benefits if both of these candidates are knocked out and the raced winnows to himself and Trump.
Polls show Rubio trailing badly in Florida, losing to Trump by more than 20 points, and possibly coming in third behind Cruz as well I expect Rubio to due better than the polls show due to ground organization and early voting, but probably not enough to win the State. Therefore, Rubio is likely going to leave the race. Kasich has been getting stronger in Ohio, with most polls showing him within a few points of Trump either way. Since Rubio has encouraged Ohio supporters to back Kasich, this has helped him surge in recent days. Kasich could finally win his first State. This may, however, be too little too late to affect the overall race.
In addition to Florida and Ohio, primaries are also taking place in Illinois, Missouri, and North Carolina. The Northern Mariana Island will also hold a Republican Caucus. Missouri, like Florida and Ohio, is winner take all. It is quite possible Trump could win these three States as well as a majority of delegates an Illinois and North Carolina, which would drastically increase his delegate lead. Even if both Rubio and Kasich drop out, Cruz would remain a distant second and has shown difficulty getting much of any support in open primaries. Trump's path still remains the odds on favorite.
On the Democratic side, all eyes are on Sanders. He remains lagging behind Clinton. After a surprise win in Michigan last week, supporters are hoping that Sanders can once again defy the polls and move that magic into other midwestern States like Illinois and Ohio. He needs to do something. The polls, however, remain in Clinton's favor. Clinton had been up 30 points in Illinois, and probably more than that in Florida. She wass also up 20 points in both Ohio and more conservative North Carolina. Missouri, which tends to be conservative seems to be the closest with Clinton up 7 points, although polling in that State seems to be weak.
Since his Michigan win last week, Sanders had appeared to be making gains in many of these States. He could possibly win Illinois and Missouri now. Sanders needs a win somewhere to keep the horse race narrative alive. Clinton is looking for four knock outs to make her look more inevitable.
Unlike the Republicans, Democrats do not move to a winner take all format. Even as Sanders loses States, he picks up some delegates. He keeps open the chance that his fortunes will change and that we will fare better in some of the later primaries. He remains a long shot. At the same time it will be difficult for Clinton to deliver a final knock out blow.