Hillary Clinton and Donald J. Trump have drawn closer to their parties’ nominations with almost every passing week. The five elections on Tuesday — in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island — may offer them their best chance before June to gain a final, decisive advantage over their opponents.

But Senator Bernie Sanders of Vermont has been a tenacious opponent on the Democratic side, and could find precincts of strength even on a Primary Day that favors Mrs. Clinton. Gov. John Kasich of Ohio and Senator Ted Cruz of Texas face even more daunting terrain on the Republican side; for them, this week is a battle for survival.

Some of the big questions we will be watching as the returns roll in:

Can Clinton effectively claim the nomination?

It would be almost impossible for Mrs. Clinton to amass enough delegates on Tuesday night to clinch the Democratic nomination, even including superdelegates. But if she manages a commanding performance across the Northeast, after her strong victory in New York last week, she could come close enough to declare the race all but finished.