House Intelligence Committee Chairman Adam Schiff said Sunday that President Donald Trump and his allies are only pushing alleged conflict of interest questions involving former Vice President Joe Biden, his son Hunter Biden and Ukraine because Trump thinks Biden "is his most formidable opponent."

The alleged Biden conflict of interest questions that Giuliani and the president have referenced involves the then-vice president's 2016 call for Ukraine to crack down on corruption, including removing a Ukrainian prosecutor, Viktor Shokin, seen as ineffective. Shokin had investigated energy company Burisma Holdings, where Biden's son was a board member.

"I don't know the circumstances in which [Hunter Biden] took the job, but I can say this vis-a-vis Joe Biden: We're providing generous support to Ukraine; we're providing defensive weapons to Ukraine; we want Ukraine to be successful in its conflict against Russia," Schiff, D-Calif., told ABC's "This Week" on Sunday. "But part of that is having a government that the people of Ukraine are willing to fight for and protect. And they've had a corruption problem. That's what Joe Biden was trying to address. So going after his son is just a method of going after someone the president believes is his most formidable opponent."

The Biden-Ukraine nexus came into focus within the past 10 days as Trump's personal attorney, Rudy Giuliani, began discussing it. On Thursday, Giuliani told The New York Times he planned to visit Ukraine soon to push the incoming administration there to move forward with investigations involving Biden's son as well as inquiries related to special counsel Robert Mueller's investigation into Russian interference in the 2016 campaign.

"We're not meddling in an election, we’re meddling in an investigation, which we have a right to do," Giuliani told The Times, which reported that he planned to ask Volodymyr Zelensky, the Ukrainian comedian elected as that country's president last month, to move ahead with the investigations.

But in an interview with Fox News on Friday, Giuliani said he was no longer planning to travel to Ukraine. News of the trip led Sen. Chris Murphy, D-Conn., to call for a Senate Foreign Relations Committee probe into Giuliani's efforts in Ukraine, citing "the implications of this for United States foreign policy" in a letter to the committee's chairman.

The Times reported that Biden’s campaign said he acted to carry out U.S. policy "without regard to any activities of his son, that he had never discussed the matter with Hunter Biden and that he learned of his son’s role with the Ukrainian energy company from news reports."

Hunter Biden said in a statement to the Times that he has had "no role whatsoever in relation to any investigation of Burisma" and "explicitly limited my role to focus on corporate governance best practices to facilitate Burisma’s desire to expand globally.”

Burisma has disputed accusations against it related to former prosecutor Shokin's investigation, the newspaper reported.

Bloomberg News, citing documents and a former Ukrainian official, reported last week that the Burisma investigation was dormant for more than a year before Biden called for Shokin to step down. PolitiFact reported it found no evidence to "support the idea that Joe Biden advocated with his son's interests in mind."

Schiff echoed that point Sunday, saying, "There's no evidence, nor has there ever been any evidence, that he was doing anything but trying to get the Ukraine government to crack down on corruption."

And while both Giuliani and the Times said Ukrainian prosecutors have reopened the Burisma investigation, a spokesperson for those investigators told Bloomberg they had not done so. That spokesperson did not respond to a request for comment from NBC News.

Pointing to Giuliani's conversations with Ukrainian officials and his planned, but then canceled, trip, Schiff said, "Don't seek the help of a foreign government" in the 2020 election.

"If this isn't criminal — and Bob Mueller said he could not prove all the elements of a crime — then maybe we need to change the elements of that crime, because we cannot make this the new norm, where if you cannot win an election on your own, it's fine to seek help from a foreign power," he said.

Elsewhere in his Sunday interview, Schiff said he didn't think the U.S. "could survive another four years with" Trump.

"He doesn't seem to understand that a fundamental aspect of his job is to try to make us a more perfect union," Schiff said, adding that he believes Trump is "going to be defeated."

"He has to be defeated because I don't know how much more our Democratic institutions can take," Schiff said.