"I don't know her. She's a very capable person. People I know have a high regard for her. But she's not a Republican," he added.
Trump argued for low rates to keep the national debt of $19 trillion somewhat manageable. Calling himself "the king of debt" in his business dealings, he warned that the national debt would be troublesome if the cost of borrowing increases.
"We're paying a very low interest rate. What happens if that interest rate goes up 2, 3, 4 points?" he asked. "We don't have a country." The U.S. should refinance longer-term debt, he added.
"We do need money to rebuild the infrastructure of the country," he said, railing against spending trillions of dollars in wars in the Mideast while American roadways, bridges, mass transit systems and airports are crumbling.
"The beautiful thing about infrastructure is it puts people to work, immediately puts people to work," he continued, "But it's got to be done properly, on time and on budget."
Trump repeated that he would renegotiate the nation's trade agreements, which he contends were hammered out by "political hacks" rather than the "brightest business people."
Echoing a common theme, he said China is killing U.S. trade by manipulating its currency. "So I'm not blaming China. I wish our people would think the same way. I blame our leadership. It's incompetent."
On immigration, Trump said his policies would not shrink the economy. In the campaign, he's talked about tough measures to make sure people come into the U.S. legally. He's also advocated building a wall along the U.S.-Mexico border.
Trump said if he doesn't win the White House, the Supreme Court would be stacked with liberals who would make the United States a "totally different country" — like Argentina or Venezuela.
Trump also gave a 40 percent chance that he would choose his running mate from his former rivals in the race for the nomination.
He said he has a good relationship with John Kasich, but the Ohio governor, who was the last to drop out, is unlikely to be his VP choice. "I not sure John even wants it."
Trump said he prefers to pick a running mate with government experience, because as a businessman he has business issues covered and he'd like help in pushing legislation through.
Sen. Jeff Sessions, who has endorsed Trump, would make a "good anything," Trump said in response to a question of whether the Alabama Republican would make a good VP choice.
"I'm now watching her fight it out [with Sanders]. She's going to win again because the system is rigged," Trump said. "[Sanders] cannot win, unless she gets indicted."
He said Clinton should be held legally responsible for using a private email sever during her time as secretary of state, but he claimed she's being protected by the Democratic machine and is going to "skate away with it." He called the process a "disgrace."
Trump said he looks forward to the general election debates. With three on the schedule, he said he wishes there were more.
Trump called Obamacare a "total disaster," saying the Affordable Care Act needs to be repealed and replaced. "The country cannot afford it. People cannot afford it. It's good for almost no one."
He said he'd get rid of a numerous regulations that he claims hurt big companies and small businesses alike. "Dodd-Frank has to be either eliminated or changed greatly," he said, adding that the regulators are running the banks.
Trump said he'd also lower taxes, particularly U.S. corporate taxes, which are higher than those of many countries.
Responding to a call by the Rolling Stones for Trump to stop using their music, the billionaire said he uses lots of different songs at his rallies. "I always buy the rights." The Stones said they have not given their permission.
Also Thursday, the Trump campaign named hedge fund veteran Steven Mnuchin as finance chairman for his general election bid. Trump has boasted that he largely self-funded his primary campaign.
Trump's remaining two rivals — Kasich and Texas Sen. Ted Cruz — dropped out of the race this week after their poor showings Tuesday in Indiana's primary.
Trump has amassed 1,055 delegates, 182 short of the 1,237 need to clinch the nomination at this summer's convention.
June 7 is the final day of primaries, with contests in California, New Jersey and three other states.