NBC and CNN both projected Romney would take Indiana from President Barack Obama. With 88 percent of precincts counted, Romney led 55 percent to 43 percent, CNN said.
The Indianapolis Star reported there were still long lines in many central Indiana communities when the polls closed.
Romney had been expected to take Indiana.
In the state's U.S. Senate race, Democratic candidate U.S. Rep. Joe Donnelly defeated Republican candidate Richard Mourdock 49-45 percent with 86 percent of precincts counted, CNN said.
Mourdock knocked off incumbent Richard Lugar in the GOP primary but got himself into hot water with his comments during a debate that he is opposed to abortion in the case of pregnancies resulting from rape because they are "something that God intended to happen."
Libertarian Rupert Boneham was in single digits.
The state Legislature is held by the Republicans, but they were looking to pick up seven more seats in the House to achieve a supermajority. The GOP held a four-seat cushion for its supermajority in the Senate.