The day broke early in Clinton's direction as she rolled to an overwhelming victory in Florida and followed that quickly with wins in North Carolina and Ohio. The first two were expected, given the makeup of the electorates in those states. Ohio's demographics were close enough to those in Michigan to give Sanders hope of a repeat victory, but Clinton's success dashed those hopes and blunted whatever momentum he had enjoyed.
The Ohio results represented a back-breaking blow to Sanders. His populist, anti-establishment insurgency has fired the energies of the party's grass-roots progressives, and there is little doubt that he has both the determination and the resources to keep fighting. His campaign has accomplished far more than almost anyone anticipated and he has shaped the issue agenda and the dialogue in the Democratic nomination contest.
For all those assets, Sanders has fallen behind in the unforgiving arithmetic of the way Democrats choose their nominees. Clinton's lead has been built by taking advantage of states where the demographics tilt heavily in her favour, particularly those with substantial populations of African-Americans, while holding Sanders close in the states he has won.
Clinton holds a lead of nearly 300 pledged delegates, those determined by the results of the primaries and caucuses. That is a bigger lead in pledged delegates than then-senator Barack Obama had in his epic battle against Clinton eight years ago.
Clinton's lead among so-called super delegates - party leaders and elected officials - is even more overwhelming.
Because Democrats award pledged delegates proportionally, Sanders needs not only a string of victories but also popular vote margins large enough to pick up delegates in bushel baskets, contest by contest. For those who have questioned the quality of Clinton's campaign, there's no doubting the effectiveness of her delegate-focused strategy.
Clinton's victory speech gave the clearest sense so far that she now feels confident about how the nomination will end. Her message was aimed at a general election contest against Trump.
She excoriated the Republican frontrunner repeatedly, drawing attention to what she called his negative and divisive campaign.
Trump's victories were another reminder of his ability to overcome adversity. For the past five days he has been on the defensive, criticised for encouraging violence against protesters at his rallies.
He also was the target of millions of dollars in negative ads in Florida. In the face of that, he swamped Rubio in Rubio's home state and won North Carolina and Illinois as well.
Trump was very much on the mind of Kasich when he appeared before supporters and promised he would not take a low road to the highest office in the land. Kasich was winless until yesterday, and Ohio was a make-or-break test. His ebullience was emblematic of his personality but also underscored Kasich's relief - and perhaps surprise - that he is now one of three remaining candidates for the GOP nomination.
Kasich's path ahead is still perilous. His belief is that the victory in Ohio will fundamentally change the dynamic of the GOP race and that the anti-Trump forces will begin to coalesce around him now that Rubio is out. But there are few states that offer obvious victories.
There is no way he can win a majority before Cleveland, and it's almost certain that he will not even be the delegate leader by the time the primaries and caucuses end. His real hope for winning the nomination is to seize the prize at the convention in Cleveland. He needs time, and enough victories, to prove his worth and then rely on a deadlocked convention to choose him to prevent Trump from becoming the nominee.
Kasich has yet to prove he is a long-distance runner. That is not the case with Cruz, who like Trump is emblematic of the strength of the outsiders in the Republican race. The Texas senator was at risk of ending the night without a victory, with his last hope coming in Missouri, where he and Trump were in a tight race. The results left him even further behind Trump in delegates.
Cruz could yet become the true challenger to Trump. His campaign has even sketched out a scenario under which he would end up with more delegates than Trump heading into the convention. But that depends on his ability to corner Trump in a one-on-one battle in the remaining states.
Some national polls have shown that when matched head-to-head against Trump, Cruz enjoys more support. That is the foundation on which the Cruz camp is building its strategy for winning the nomination. But that strategy suffered a setback yesterday when Kasich won Ohio.
