Correspondents say Mr Trump will be virtually unstoppable if he wins there.
Rival Ted Cruz said he was relying on Indiana voters to turn the tide of the election.
Meanwhile in the Democratic battle, Hillary Clinton holds a commanding lead over Bernie Sanders.
Mr Trump is hoping that a victory in Indiana will put him within reach of the 1,237 delegates needed to guarantee him the Republican nomination, and allow him to turn his attention to the general election.
“You know if we win it’s over, and then I can focus,” he said at a campaign rally in the state on Monday night.
Mr Cruz cannot get enough delegates to win the nomination outright, but hopes he can still deprive Mr Trump of a majority.
He has thrown considerable resources into the state in recent days.
Polls show Mr Trump has a commanding lead, but Mr Cruz said he felt confident voters would back him.
“Indiana is in a position to decide the direction of this race, not just for Indiana, not just for the Republican party, but for the entire country,” he told supporters in the state.
“I think… the country is depending on Indiana to pull us back from this cliff.”
The third Republican candidate, John Kasich, is no longer campaigning in Indiana.
He and Mr Cruz reached a deal last month to coordinate their strategies against Mr Trump, a move which prompted Mr Trump to label them “desperate”.
In the Democratic race, poll suggests a comfortable victory for Mrs Clinton over Mr Sanders.
She currently has 91% of the delegates needed to win the nomination.
Source: 3news.com|Ghana
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