March 1st, or Super Tuesday, confirmed the breadth of Donald Trump's support in the Republican nomination race: he won in the moderate north-east, the evangelical south and in the conservative Midwest. March 15th is a test of his durability. Since Super Tuesday, his rivals have attacked him in the way that they should have done since the beginning of the race. His business record, his apparent racism and ambiguous policies have come under greater scrutiny. But none of these punches have connected properly, and nor are they likely to, at this comparatively late stage in the race. Mr Trump's appeal as an anti-establishment, free-speaking, in-hock-to-no-one renegade is only being enhanced by the criticism from the heart of the Republican Party.
The Economist Intelligence Unit expects Mr Trump to win the Florida primary, and with it all of the state's 99 delegates. He has a lead in all 19 of the major polls conducted in the state in March, by a margin of between 6 and 25 percentage points. Although the Democratic primary in Michigan was a useful reminder that polling can occasionally be wrong, there is nothing in the campaign of his main rival, Marco Rubio, to suggest that he is likely to unseat Mr Trump.
The other big prize up for grabs is Ohio, the home state of governor John Kasich, the fourth man in the race. Ohio has 66 delegates and also awards them all to the winner. As much as Florida, the outcome in Ohio is crucial to the destination of the nomination. Polling in Ohio has indicated that Mr Kasich has pulled ahead of Mr Trump in recent days, and we expect home advantage to prevail. Mr Kasich has devoted a big proportion of his resources to Ohio, which he hopes to use as a springboard for victories in other, more moderate states.
Should voting pan out as we expect, with Mr Trump also winning the largest share of the delegates in Illinois, North Carolina and the Northern Mariana Islands, and all of them in a winner-takes-all contest in Missouri, he would be up to around 690 delegates. This means he would need around 550 of the remaining 927 delegates on offer in the remaining states to secure the nomination. This is a high bar if all of the other three candidates remain in the race. However, if, as is widely expected, Mr Rubio ends his campaign after failing to win Florida, Mr Trump would be a big beneficiary. (The same applies to John Kasich if he loses in Ohio.) Thus the March 15th primaries are not just about delegates, but also the decisions taken afterwards.