OTTUMWA, Iowa — Republican presidential candidate Donald Trump escalated his criticism of GOP rival Sen. Ted Cruz (Tex.) here in Iowa Saturday, painting him as a political follower beholden to pro-oil special interests and the donor class.

Trump’s strategy: to raise further questions about Cruz’s stance on ethanol— an important industry for voters in the Hawkeye State — in the final weeks before the Feb. 1 caucuses. Cruz has faced renewed scrutiny over his opposition to the 2005 Renewable Fuel Standard, which requires gasoline to be blended with amounts of corn ethanol and is set to expire in 2022.

“As you know, my primary opponent in Iowa — only in Iowa, because Ted actually isn’t do very well in New Hampshire, but in Iowa he’s doing well — was totally opposed to ethanol and the ethanol industry because he’s with the oil industry. He’s from Texas, I guess that makes sense,” Trump said.

The real estate mogul’s criticism comes amid an escalating battle between the two candidates for the top spot in polls of Iowa voters. A Fox News survey of Iowa voters had Cruz leading Trump among likely GOP caucusgoers, 27 percent to 23 percent.

While touting his own support for the industry, Trump said he believes Cruz’s position on ethanol has undergone “a very big change.” Cruz has faced scrutiny in recent days for his one-time opposition to the RFS; he has softened his position in recent years, instead calling for legislation that would gradually phase out the requirement.

“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. Nobody really believes it," Trump told the audience.

Cruz has denied changing his position. His campaign released a statement Wednesday saying, “Cruz has consistently supported a five-year phase out of the Renewable Fuel Standard" and "first introduced the five-year phase out in 2014."

Supporters and spectators waited outside of the Bridge View Center here in Ottumwa — a small town in southern Iowa — where a harsh cold wind blew as they waited for a chance to hear one of Trump’s notoriously raucous speeches. Though the crowd capacity in the auditorium was about 650, according to police officers on site, the overflow area held about 1,000 people who stayed to listen to a short speech by Trump after the event.

Trump’s message resonated with many supporters at the rally, who voiced intense anti-Washington sentiment in interviews with the Post. Many of them described Cruz, whose national identity was forged by his frequent battles with establishment Republicans, as an indecisive politician willing to change his positions when it is political expedient.

“Ted seemed to try to play the political game. I think he has similar ideas, he’s a closet Trump,” said Kermit Smith, 51, who grimaced when Cruz’s name came up. “And now that Trump’s ratings are going up it’s almost like Cruz is chasing after him, riding on his coattails.”

Trump “is the only one of the candidates on either side that has answers to questions,” Smith said. “Most of the others candidates just say, 'Oh, that’s a difficult and complicated problem.'"

But even though there is tremendous excitement surrounding Trump, who is far and above the national party front runner, what remains to be seen is if his campaign will be able to channel that energy into a strong showing at the caucuses. Several voters in attendance said they liked Trump’s ideas but were not committed to actually voting.

“He’s about it for me. I’ve never been to one of these before but I really like his ideas,” said Michelle Turner, 44, of Ottumwa. But when asked if she intended to caucus, her answer was direct: “No, nuh-uh.”

Trump joked that he would be in Iowa so much in the coming weeks that voters are "going to be so sick of me." he urged voters to show up to the caucuses to "do what's right."

“Iowa, you know you haven’t been good at picking the winners, folks. We’ve got to pick a winner this time,” Trump quipped. “I’ll be saying that the day later, ‘They haven’t picked a winner in years!’ But you know what, if you pick me you’re going to pick a winner.”

“We’ve got to win Iowa, oh we’ve got to win it,” Trump said later, sighing heavily. ”Otherwise we’re wasting our times. … Don’t sit home. Don’t sit home. Get out there, it’s so important.”

After finishing his remarks in Ottumwa, Trump immediately left for an event in Clear Lake in the northern part of the state. There, he raised more questions about Cruz’s Canadian birth, which he has repeatedly mentioned as a potential obstacle for the Texas senator in recent days.

“We know the Democrats are going to bring a major suit," Trump told the crowd. “If you're born in Canada it's immediately a little bit of a problem.”

Trump added that “I don’t want to win this way, I want to win fair and square” but that Cruz should nonetheless seek a declaratory statement from a judge to clarify his eligibility.

“You’ve gotta have the courts come up with the ruling or you have a candidate who just cannot run,” Trump said. “Who knows more about lawsuits than I do?” he said. “I’m the king. … You can’t have somebody running and have a lawsuit.”

Cruz dismissed Trump's comments during a campaign stop in Waverly, Iowa. As he has whenever the issue has come up, he said that despite being born in Canada he was automatically granted American citizenship because his mother was American, saying it was "the process of being born that made me a citizen."

He did not take on Trump directly, but noted that he is coming under increasing fire from the other Republicans as his poll numbers rise. He called it "political noise" aimed at getting the media to embark on "rabbit trails and circus side shows."

Karen Tumulty contributed to this report.