Here are all the states where you have to register as a Democrat to vote in the primaries

But states all have different policies about who can vote in the primaries, and whether or not you have to be registered with a given political party to vote in their primary.
In states with closed primaries, you must be registered as a Democrat to vote in the Democratic presidential primary.
States with open primaries, on the other hand, allow unaffiliated voters, people registered to different parties, or both, to vote in the Democratic primary.
Here's which states have open and closed Democratic primaries, and the deadline to register in every state.
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The first primary contests of the 2020 election are just under three months away, and the deadlines to register to vote are coming up in several early states.

Over the course of almost four months between February and June, nearly every US state and territory will hold a primary election or caucus to determine how they will allocate their delegates to determine the 2020 presidential nominees for each party.

But states all have different policies aboutwhocan vote in the primaries, and whether or not you have to be registered with a given political party to vote in their primary.

In states with closed primaries, you must be registered as a Democrat to vote in the Democratic presidential primary. Some states with closed primaries, like Oregon and Connecticut, technically have closed primaries but let their state's Democratic Party decide whether or not to allow unaffiliated voters to cast ballots.

States with open or partially open primaries allow unaffiliated voters, and in some cases, people registered to different parties, to vote in the state's Democratic presidential primary.

According to Green Papers' data , 23 states have fully or partially closed Democratic presidential primaries, meaning you must be registered as a Democrat to vote in the primary.

Eleven states further have partially open primaries where unaffiliated but not registered Republicans can vote in the Democratic primary. And sixteen states have fully open primaries where any person, regardless of their party registration, can cast a ballot in the Democratic presidential primary, however, voters can only participate in one party's primary.

Keep in mind, however, that many states have different rules for federal and state-level primary elections, and this map of which states have open or closed primaries only applies to Democratic presidential primaries.

And since every state has its own unique laws and stipulations regarding presidential primaries, double-check your own state's rules before deciding how to register.