Trump knocked 11 points off of Hillary Clinton's lead in one week.
“A whole generation is being taught by professors who voted for Bernie Sanders,” he said.
It's a state a Democratic presidential candidate has only carried once in 64 years.
The polls shows voters expect the two parties' front-runners to face off in the general election.
Indiana is an important test for Trump in his quest for 1,237 delegates.
Clinton's lead is down from March, but she still posts a 22-point advantage
Clinton leads by 2 points in a state Republicans have won in seven of nine presidential elections.
Two national polls found her lead over Sanders is narrowing.
But the pollster warned that Trump's support has slipped in recent days.
Poll comes as Trump drops plans to attend CPAC to campaign in Kansas.