Libertarian Party United States
The congressman had planned to run as a Libertarian to more easily get on state ballots in the 2020 election.
The former governor of Rhode Island mounted a short-lived run for the Democratic presidential nomination in 2015.
The former GOP governor said Republican leaders "want to have no election... I donât think that would be very good."
The move makes Trump the first incumbent president since George H.W. Bush in 1992 to face a notable primary challenge.
WHAT'S HAPPENING
It's now or never!