Donald Trump called Evan McMullin the “puppet” of a “loser” on Saturday, prompting the third-party candidate who threatens to beat Trump in Utah to retort that the Republican nominee was “deeply corrupt”.

McMullin launched his quixotic presidential campaign in August, presenting himself as an alternative for conservatives who do not want to vote for Trump. A Mormon and former counter-terror operative with the CIA, McMullin has shot into competition in his native Utah.

He is only a few points behind Trump in the deeply religious state, where many Mormons have rejected the businessman in the wake of accusations of harassment and sexual assault.

In an interview with Fox News on Saturday night, Trump directed his ire toward McMullin, whom he had previously ignored. “He takes votes away from me, this man who I never heard of,” he said.

His running mate, Mike Pence, said the same: “Nobody ever heard of him.”

“He’s going from coffee shop to coffee shop,” Trump continued, apparently alluding to local campaign stops. “It’s a full-time thing. He can’t do anything. But he hurts us in Utah.”

Trump then blamed Bill Kristol, a conservative pundit with whom the businessman has feuded for more than a year, for bringing a potential spoiler into the race.

“He’s a loser,” he said. “He’s called me wrong. He’s called everything wrong. He’s called the wars wrong. Everything he does is wrong. He gets his puppet to go and run in Utah. The guy takes votes away from me.”

Trump argued that McMullin could deliver a national victory to Hillary Clinton, costing Republicans the ability to approve supreme court nominees – the issue of most concern to many wavering conservatives.

“That means we’re probably going to lose the supreme court of the United States for 60 years,” he said. “By that time it won’t matter, because we won’t have a country left.”

McMullin replied on Twitter, saying: “You’ve never heard of me because while you were harassing women at beauty pageants, I was fighting terrorists abroad.

“You’re an authoritarian, life-long liberal who disrespects life, liberty [and] equality,” he continued. “We can’t trust you on justices. You think you’re entitled to Utahns’ and other Americans’ votes. We’re earning them.

“We will not sit by while two corrupt, self-serving, big government liberals like you and Clinton destroy our country.”

On Sunday, McMullin continued to attack.

“This is a man who has unhealthy relationships and admiration for dictators overseas,” he told NBC’s Meet the Press. “I believe that he has some of the same ideas about leadership or domination that they do. I think he’s dishonest with the American people.

“Where are his tax returns? We don’t know much about Donald Trump.”

He went on to call Trump “unethical”, “dangerous to our country” and “somebody who doesn’t understand the constitution”.

McMullin acknowledged that he had no path to the 270 electoral college votes necessary to win the presidency, and instead said he hoped for the unlikely scenario of stopping the major party nominees reaching that target, which would send the election to the House of Representatives.

He did not answer questions about whether he felt comfortable knowing his success in Utah could help Clinton win the election and said he remained “very skeptical” that the Republican party could be reformed after its marriage to Trump.

He also said that he has received no financial support from Mitt Romney, the party’s 2012 nominee and a fellow Mormon who has emphatically denounced Trump.

“We would welcome it,” McMullin said. “Unfortunately, it’s not the case.”