Voters go to the polls in four states on Tuesday, with Michigan the biggest prize for both parties. Donald J. Trump seeks to strengthen his position as the Republican front-runner, while his rivals look to slow his drive toward the nomination. For the Democrats, Senator Bernie Sanders of Vermont faces a crucial test in his upstart campaign to derail Hillary Clinton. Here are some of the things we will be watching in the contests in Hawaii, Idaho, Michigan and Mississippi.
Is Trump Fading?
Mr. Trump had a rough week. He faced attacks from the party establishment and criticism for his debate performance on Thursday before barely outpacing Senator Ted Cruz of Texas on Saturday in Kentucky and Louisiana, and losing to him in Kansas and Maine, where Mr. Trump was considered a favorite.
But it is not clear whether he struggled to win because he had lost ground or because anti-Trump voters had consolidated around Mr. Cruz. Mr. Trump's share of the vote on Saturday was roughly in line with what he had won on Super Tuesday; Mr. Cruz finished with a far higher share of the vote than his Super Tuesday total.