Mr. Trump is also making an aggressive push to win the state. He planned to appear at least one more time with Mr. Knight, this time in Evansville, the largest city in southern Indiana. And he is deploying additional staff members here: His top aides told a small group of Indiana Republicans at the Republican National Committee meeting last week that Mr. Trump already had 20 campaign workers on the ground in the state and planned to bring in 20 more.

“I am here for three days, then coming back for three more,” Mr. Trump said Wednesday evening, before staying overnight at a Holiday Inn Express, according to a message posted hours later on Twitter. “I’m not playing games with Indiana.”

Unlike in Wisconsin, the last state where he and Mr. Cruz made such an aggressive push, Republican leaders here have yet to rally around one of the candidates. Gov. Mike Pence; his predecessor, Mitch Daniels; and Senator Dan Coats have all stayed out of the race. Some of Indiana’s establishment-aligned Republicans drifted to Mr. Kasich, but his semi-withdrawal from the state has even further complicated the situation for mainstream conservatives.

“I have no idea if I’ll vote for a presidential candidate now,” said Jim Merritt, a state senator from the Indianapolis area who had been inclined to back the Ohio governor. “I am very disappointed.”

Perhaps the most consequential question looming over the race now that Mr. Kasich has pulled back is whether his voters will support Mr. Cruz to stop Mr. Trump. It is especially pivotal in the Indianapolis area, the most voter-rich and affluent part of the state. If Mr. Cruz can make a strong showing here, it could offset Mr. Trump’s expected strength in the industrial areas of northern Indiana and the more working-class precincts of the state’s south.