Washington (CNN) Wisconsin Republican Sen. Ron Johnson said Thursday morning that it's too early to say whether he'll support a re-election bid for Donald Trump in 2020, adding to a growing list of conservatives on Capitol Hill who are holding out their endorsements for their party's President.

"It's way too early to be talking about 2020," Johnson told CNN's "New Day" co-anchor Chris Cuomo. "It could be a completely different world by 2020. We have a 2018 election first."

JUST WATCHED Some Republicans not ready to back Trump in 2020 Replay More Videos ... MUST WATCH Some Republicans not ready to back Trump in 2020 02:32

Johnson called the prompt for Trump's endorsement a "gotcha question," adding that he thinks "it's unusual that members of the media would be talking about the 2020 election in 2018."