DES MOINES, Iowa — Donald Trump arrived in Iowa Saturday with his eyes on the prize and Ted Cruz in his sights with a little more than three weeks until the Iowa caucuses.

In events in Ottumwa and Clear Lake, Trump didn't hold back on the Texas senator, who continues to maintain a lead in the Hawkeye State. Trump took swings at Cruz over issues including his stance on ethanol and status as a natural born citizen. The stops were Trump's first of 2016 in the state.

"My primary opponent in Iowa — only in Iowa because, you know, Ted isn't doing very well at all in New Hampshire — but in Iowa he's doing well. And my primary opponent was totally opposed to ethanol and the ethanol industry because he's with the oil industry," Trump told the Ottumwa crowd. "He's from Texas, I guess that makes sense because he's with the oil industry."

"He was getting clobbered and all of a sudden he said, 'Uh, oh, I'm for ethanol.' You can't do that. You can't do that. With three weeks to go you're not allowed to do that. So nobody really believes it," Trump said.

Trump's attacks come amid questions raised over the past week regarding Cruz's opposition to the 2005 Renewable Fuel Standard, which his campaign has tried to tamp down throughout his six-day bus tour that wrapped up Saturday.

Since 2014, Cruz has called for winding down the program over five years by slashing it 20 percent each year. The law, which is set to expire in 2022, requires that gasoline be blended with certain amounts of corn ethanol. Trump worked to raise doubts in the minds of Iowans about Cruz's view with only 23 days until voting kicks off.

Trump saved his sharpest barbs for questioning Cruz's natural born citizenship. Trump says Democrats will raise the issue if the Texas senator snags the Republican nomination.

The Constitution requires that the President be a "natural born citizen." Cruz was born in Canada, but because he mother was an American citizen born in Delaware, most scholars believe he is eligible for the presidency.

After touching briefly on the topic in Ottumwa, Trump put his foot on the pedal in Clear Lake, arguing Cruz simply is not a natural born citizen and that he needs to have the courts rule on his situation.

"You can't have a person who's running for office, even though Ted is very glib and he goes out and says 'Well, I'm a natural born citizen,' but the point is you're not," Trump railed. "You've gotta get a declaratory judgment. You have to have the courts come up with a ruling. Or you have a candidate who just cannot run. Because the other side will immediately bring suit and you've got that cloud on your head, and you can't have that cloud on your head."

"[Y]ou cannot put somebody there, folks, who's going to go in, and he's going to be immediately sued by the Democrats because they're saying he was born in Canada and he's not allowed to run for president," Trump told the crowd, adding that he always thought one had to be born in the U.S. in order to run for president.

The billionaire also bashed Cruz for taking millions in donations for his super political action committee, pointing indirectly to the $15 million Cruz's PAC has received from Farris and Dan Wilks.

"Frankly, Ted Cruz has two guys that have given him a fortune," Trump said. "One is oil and one is something else. Remember ethanol, you're not protected. I guarantee you that. I guarantee you that, but Ted has guys."

According to the latest Fox News poll, Trump trails Cruz, 27 percent-23 percent, with Florida Sen. Marco Rubio posting a strong third with 15 percent. No other candidate is in double digits. Cruz also leads Trump in the Washington Examiner's presidential power rankings.

Trump is set to return to Iowa Tuesday with a rally in Cedar Falls slated for 7 p.m. EST Sunday.