There is no party registration in Montana. Individuals who vote in a primary election are given all the parties' ballots, and can choose in private which party ballot they wish to vote.

Tuesday 2 June 2020: Montana Primary. Hours 7:00a-8:00p MDT (1300-0200 UTC). A polling place with less than 400 registered voters must be open at least from noon to 8 p.m. or until all registered electors have voted. Delegate Selection: Winner-Take-All Primary. Voter Eligibility: Open Primary.

Primary
Source: Montana Secretary of State.
Status: Official. Retrieved: Sunday 19 July 2020.
Included in Nationwide vote.

Montana Presidential Primary. The candidate receiving the most votes receives all of Montana's 27 delegates.

For purposes of the 2020 Republican National Convention, the winner of the primary election ... shall select a slate of delegates ... from the pool of National Delegate/Alternate applicants.... [Section B: III. F) 3)]

The 3 party leaders, the National Committeeman, the National Committeewoman, and the chairman of Montana's Republican Party, will by virtue of their position, attend the Republican National Convention bound to vote for the winner of the Montana Republican Primary.

All of Montana's delegates ... are bound to vote for the winner of the Montana Republican Primary ... through the first ballot, if that candidate's name is placed in nomination.

Delegates to the State Delegate Convention are elected from each county by the County Central Committee at least 10 days before the State Convention. [Montana Republican Party Bylaws. Section B - III. D]

Thursday 11 June - Saturday 13 June 2020: State Convention / State Delegate Convention in Missoula. The convention slates the state's Republican National Convention delegates and nominates Presidential Electors. [Section B - III. B. and F. 1 through 7)]