By Eli Watkins, CNN

(CNN) -- A potential GOP primary challenger to President Donald Trump explained further on Monday why he was stepping in and why he was concerned about Trump's leadership.

Referring in an interview with CNN's Alisyn Camerota on "New Day" to Trump's 2017 Oval Office meeting with Russian Foreign Minister Sergey Lavrov and then-Ambassador Sergey Kislyak, former Massachusetts GOP Gov. Bill Weld said, "That showed contempt for the American people if anything I've ever seen does."

Weld cited the moment as one that stuck out in his mind as he gears up to mount a long-shot primary bid to challenge Trump for the Republican presidential nomination after announcing an exploratory committee last Friday.

Weld said the response to his announcement had been "for everybody to close ranks among the state Republican parties" and limit the ability for a Republican to challenge Trump, and he argued his path forward would have to include persuading independent voters to step into GOP primaries to side with him over Trump.

"There's a very conservative group, not my base for sure," Weld said. "There are 20 primaries where independents and un-enrolled voters can come in and vote in the Republican primary. Those will have my attention."

Weld was on the ballot in 2016 as the Libertarian Party's vice presidential nominee. He was a regular critic of Trump on the campaign trail as he promoted his third party effort led by Libertarian presidential nominee Gary Johnson. At the time, the former Republican governor pledged lifelong loyalty to the third party, telling them he would "stay with the Libertarian Party for life."

Asked on Monday why he was not opting to stay with the Libertarians this cycle, Weld said he wanted to make a direct challenge to Trump.

"I thought about the Libertarian Party again, but I decided I really wanted to go mano a mano with Mr. Trump," Weld said.