Senate Majority Leader Mitch McConnell on Monday warned against foreign interference in U.S. elections, threatening that nations seeking to meddle in the 2020 races will “have a serious problem” on their hands.

“Look, 2018 was a big success story,” the Kentucky Republican told CNBC, praising the Trump administration’s efforts to safeguard last year’s midterm elections.

“I’m convinced we're ready for 2020,” he continued. “Any foreign country that messes with us is going to have a serious problem in return.”

Critics have branded the Senate leader as “Moscow Mitch” in recent months for his refusal to to bring election security legislation to the floor. But facing sustained political pressure, McConnell two weeks ago threw his support behind a measure to appropriate an additional $250 million toward assisting states with improving their voting systems and countering foreign interference.

McConnell’s remarks Monday come as President Donald Trump enters the second week of a political crisis triggered by his July phone call with Ukrainian President Volodymyr Zelensky.

During that conversation, Trump urged Zelensky to work with Attorney General William Barr to investigate former Vice President Joe Biden and his son. Biden is one of the leading candidates for the 2020 Democratic presidential nomination, making him one of Trump's primary political rivals.

The call is the main focus of an impeachment inquiry announced by House Speaker Nancy Pelosi, but McConnell told POLITICO in a statement last Wednesday that congressional Democrats had “overplayed their hand.” He did not, however, defend the president’s behavior.