Weld, the 2016 Libertarian vice presidential candidate, said he would have been "ashamed" if he passed on running against the president for the Republican party nomination.
"I'd be ashamed of myself if hadn't raised my hand and said count me in," he told ABC news shortly after his announcement. "I think that the president is a very divisive force and that's just not my style and it's not what Americans deserve."
Weld said he’s jumping into the race not just because of Trump's tone and political tactics, but because of his refusal to address the major issues impacting Americans.
"The president is just not dealing with serious issues such as global warming and climate change. That's a real threat to us as a country," Weld said. "And for the president to just say it's a hoax, that's not responsible government."
"It is time for patriotic men and women across our great nation to stand and plant a flag," Weld said in a statement. "There is no greater cause on earth than to preserve what truly makes America great. I am ready to lead that fight."
In February, Weld told ABC News Chief Global Affairs Correspondent Martha Raddatz on "This Week" that he was was considering a primary challenge because "we don't have six more years of the antics frankly, for want of a better word, that we've seen the last two years. I think that would be bad for the country and I don't care who knows it."
On Monday, the former two-term governor boasted of his bipartisan record and ability to court independent voters.
"There's plenty of play, there's plenty of play here," Weld argued when asked about how he plans to chip Republicans away from Trump, who's historically popular with his party.
But Weld says he’s "confident" he can lure enough Republicans into his camp to make the primary a serious fight for the president. "What you see now is the president is popular with the state party apparatuses and they work with him and he's giving them orders saying, 'Don't let any fresh air in here. We don't really want a primary anywhere.'"
"The fact that we have a whole year yet to go, before even the early primaries, that's plenty enough time to make this case," Weld added.
And according to Weld, his success in 2020 will be hinged on primary states where independents can vote will be key.
"There are 20 primaries where independents can vote in the Republican primaries, 20 states, and I'll be focusing on the," Weld said. "It's not just New Hampshire. I'll probably be making swings through California, Oregon, Washington … I'll be active in the Mid-Atlantic states, particularly New York, also Pennsylvania and Delaware and Maryland and a bunch of other states out West."
And with the release of the redacted version of special counsel Robert Mueller’s report looming this week, Weld echoed Former FBI Deputy Director Andrew McCabe in saying "he may not be a Russian asset but he might as well be."
The Republican challenger to Trump argued that the president’s attacks on NATO were dangerous and exactly what Russian President Vladimir Putin couldn’t imagine "in his wildest dreams."
"NATO is our forward arm to contain the Soviet Union," Weld said. "[Trump’s] not a student of history, but he should understand at least that much."