House members sparred over the impeachment investigation of President Donald Trump at a hearing Friday as they debated how to prevent foreign interference in the 2020 election.

Opening a House Judiciary Committee hearing on election security, panel Chairman Jerrold Nadler took aim at Trump’s request that Ukrainian President Volodymyr Zelenskiy investigate Democrat Joe Biden. The revelations are shaping up as an even more serious threat to Trump’s presidency than the special counsel investigation of Russian interference in the 2016 election.

“President Trump has refused to acknowledge Russia’s attack, let alone publicly denounce it or outline clearly how he intends to deter future interventions,” Nadler said. “Now we have evidence that the president of the United States asked a foreign leader to interfere in our next election.”

Representative Matt Gaetz, a Florida Republican and ally of the president, said he worried that attempts to impeach Trump represent a bigger threat to democracy than election interference.

“I fear that the greatest risk to our democracy may not be hackers or interference with the vote, it may be the efforts by radical Democrats to try impeach a president who was duly elected,” Gaetz said.

Digital security experts told the committee that the U.S. still faces state-sponsored cyber attacks from countries such as Iran, Russia, China and North Korea.

“We must take bold, decisive and expeditious steps to address cyber threats,” Debora Plunkett, a cybersecurity expert at the Harvard Kennedy School of Government, said.