Voters in five north-eastern US states will cast ballots in a series of primaries that could cement the leads of presidential frontrunners Hillary Clinton and Donald Trump.
Democrat Mrs Clinton and Republican Mr Trump hold sizeable leads in all five contests, according to recent polling.
Mr Trump’s rivals have already shifted focus away from the north-east.
Ted Cruz and John Kasich have teamed up to help each other in the Indiana, Oregon and New Mexico primaries.
Mr Trump has condemned their pact as a sign of weakness and desperation.