People in four US states are voting in the party contests to choose the candidates who will compete in November’s presidential election.
The biggest prize is Michigan where the front-runners – Donald Trump for the Republicans and Hillary Clinton for the Democrats – will seek to consolidate leads over their respective rivals.
Both parties are also holding primaries in Mississippi on Tuesday.
In addition, the Republicans are voting in caucuses in Idaho and Hawaii.
After the previous round of voting on Saturday, Mr Trump remained well ahead of his Republican rivals, despite defeats to Texas Senator Ted Cruz in Kansas and Maine.
Mr Trump, a New York property tycoon, won in Louisiana and Kentucky. Other Republican contenders Marco Rubio and John Kasich made little progress.
Analysts say Mr Cruz appears to be the only candidate who can stop Mr Trump, who has been fiercely criticised by the Republican establishment.
On the Democratic side, Mrs Clinton lost to Bernie Sanders in Kansas and Nebraska, but won in Louisiana.
The advantage of the former secretary of state among pledged delegates – awarded according to how individual states have voted – is substantially bolstered by the hundreds of so-called “super delegates” who have said they will back her at the convention.