States including Montana and New Jersey are voting.
Four more states and Washington, D.C., will vote in the 2024 presidential primaries on Tuesday.
President Joe Biden and former President Donald Trump are the only major candidates still in the running, having both won their respective parties' nominations earlier this year.
Montana
Polling stations will be open across the state from 7 am to 8 pm.
Montana voters do not have to register with a political party and can vote in any primary they choose.
The state doesn't allocate delegates in the Republican presidential primary, but instead its 31 delegates will attend the party's national convention this summer and vote for the candidate of their choice.
Twenty delegates are at stake in the Democratic presidential primary, to be apportioned proportionally based on Tuesday's results.
New Jersey
Voting is open from 6 a.m. to 8 p.m.
The Republican presidential primary features 12 delegates up for grabs, one of the fewest in the party's nomination process.
The Democratic primary will select 126 delegates.
New Mexico
Voters can vote in person or by mail, but all ballots must be received by local officials by 7pm on Tuesday.
Early voting began on May 7th.
The Republican primary has 22 delegates at stake, while the Democratic primary has 34 delegates at stake.
South Dakota
Voting is open from 7 a.m. to 7 p.m.
Absentee ballots must be collected by county clerks by the end of the business day on Tuesday. Voters may also submit their absentee ballots in person to their county clerks.
South Dakota elects 29 delegates to the Republican Party and 16 delegates to the Democratic Party.
Washington DC
The Democratic presidential primary will be held in Washington, DC, on Tuesday.
Voters can vote by mail, but their ballots must be returned by Tuesday or brought in by 8 p.m. that day.
Polling stations will be open from 7 a.m. to 8 p.m. on the day of the primary election.
The US capital held the Republican presidential primary in March, where Nikki Haley defeated former President Donald Trump, winning two elections and becoming the first woman to win the Republican nomination.
The winner of the Democratic election will be awarded 20 delegates.
