The small northeastern state, home to just 1.3 million, is the battleground that could shake out a crowded Republican field of candidates, pitting Trump and arch-conservative Senator Ted Cruz against more establishment candidates led by Senator Marco Rubio.
The high number of registered independents, who can choose to vote either Democrat or Republican, and the up to 30 per cent of undecided voters in the final days mean everything is in play.
"This is crunch time," the Republican frontrunner Trump told thousands of cheering supporters at an event delayed slightly by snow in Manchester.
"You have to go out, you have to vote, we have to celebrate tomorrow evening," he said.
"Let's have a big, big victory."
Trump has energized broad swaths of blue collar and generally less educated Americans, angry about economic difficulties and frustrated at what they see as their country losing its stature in the world.