Appearing Tuesday on CNN’s AC360, former Republican Ohio governor and network contributor John Kasich confessed he does not see a path to defeat President Donald Trump for the 2020 Republican White House nomination.

Kasich, one of 16 candidates who lost the 2016 Republican primary to then-candidate Donald Trump, conceded to guest host John Berman that he doesn’t see a path “right now,” but suggested an opening to win the nomination may present itself down the road.

“But right now, I don’t see it,” said Kasich, a staunch Trump critic who refused to endorse him for president. Asked why the former governor believes defeating President Trump would be a towering task, he said the reason is “complicated” and cannot be explained “in a few seconds” on cable news.

Kasich has previously left the door open to launching a primary challenge to President Trump, telling CNN’s Newsroom in April that “all of my options remain on the table.”

“I don’t wake up every day looking at polls or thinking about me and my political future. I just want to be a good voice,” he said when asked about a possible run.

Kasich is not the only GOP voice who thinks he shouldn’t seek the White House. Former House Speaker John Boehner (R-OH) said in April that it would be a bad idea for Kasich to primary the president. “No, I wouldn’t — I wouldn’t bother if I were him,” Boehner replied when asked about a possible challenge in an interview on NBC host Chuck Todd’s podcast Chuck Toddcast.

Kasich first took on Trump in July 2015, when the then-governor announced his bid for the Republican presidential nomination. He dropped out of the race the following year after Trump emerged as the presumptive GOP nominee and did not endorse Trump for the general election.

In April, former Massachusetts Gov. Bill Weld launched the first Republican to challenge President Trump in the 2020 race and was followed months later by former Illinois congressman Joe Walsh. Mark Sanford, a former South Carolina congressman, is considering jumping into the Republican primary, as well.

President Trump on Tuesday mocked the trio of Republicans, writing in a tweet, “Can you believe it? I’m at 94% approval in the Republican Party, and have Three Stooges running against me.”

“I should be able to take them!” he added.