Voting in the Republican and Democratic primary contests is about to close in two states -- Mississippi and Michigan.
Polls suggest Hillary Clinton will extend her lead over Democratic Party outsider Bernie Sanders, and Donald Trump will likely continue his dominance on the Republican side.
In Michigan, turnout on the Democratic side has been so high there are reports some locations have been turning people away after running out of ballots. Headlines in the northern state have recently been dominated by the Flint water crisis, which has seen thousands exposed to lead and a number of cases of Legionnaires' disease.
"I don't think I have ever left a room as shattered as I was listening to the pain of what was going on in Flint," Mr Sanders told supporters at a rally in Warren, Michigan. He called on Governor Rick Snyder, a Republican, to resign over his "dereliction of duty".
"I could not believe I was listening to people living in the United States of America in 2016. This sounded like people in a third-world, desperate country."
Despite trailing Ms Clinton 502 delegates to 1134, Mr Sanders is confident his grassroots support will see him through.
"The people of this country are tired of establishment politics and establishment economics. They want real change in this country, and I feel very strongly… that we've got a lot of momentum and we have a path to victory."
Mr Trump leads his closest rival, Ted Cruz, 384 delegates to 300. But needing another 853 to lock in the nomination out of a remaining 1585, and with Marco Rubio and John Kasich refusing to give up, it's looking increasingly likely none of the Republican field will secure enough delegates to wrap up the nomination before the party's convention in July.
If that happens, anyone could be chosen -- even unsuccessful 2012 candidate, Mitt Romney, who hasn't ruled out accepting the nomination, should his party ask.
"I don't think anyone in our party should say, 'Oh no, even if the people of the party wanted me to be president, I would say no to it.' No one is going to say that," he told NBC.
Mr Romney recently slammed Mr Trump as a "fraud" and a "phony", but has ruled out actively running for the Republican nomination.