Do I have to register to vote for midterms if I am already registered to vote for presidential election in the US?
I voted for the presidential elections in 2016, do I need to register to vote for the midterms?
You do not. If you have not moved since you last voted, then you can simply return to the precinct where you voted in the presidential election on midterm election day to cast your vote. The Wiki-how on voter registration specifies:
You only need to register again if you change your address or name, or if you would like to switch political party affiliation. In most states, you can change this information online or by filling out a new national form. If you have any questions, contact your local election office.
Vote.gov offers information on how to register to vote. On that page, you choose your state and you can register online.
If you don't need to register again, that should be stated on the state-specific site. For example on the Arizona site, it says:
Use EZ Voter registration to:
Register to vote for the first time
Update your address, political party preference, permanent early voting status or any other information on your voter registration record
To check the status of your voter registration, click here
To find your polling place, click here
Therefore, I suggest going to the state-specific site (via the link on vote.gov) to check the rules that apply in your state. If it works like the Arizona site, you can check your voter registration, register online and find other relevant information.
There is one scenario under which you must re-register. In many jurisdictions, you are allowed to cast a ballot in a presidential election if you are registered to vote in the state, even if it is not at your current address or precinct. You are then not permitted to vote on other races.
In midterm elections, this protection will not apply. So if you are registered to vote in one place, but actually live in another, you should re-register so that you can vote at the place you should (probably close to where you live) and on the races you should (the people who will be representing you, rather than the people who will be representing the place you are registered).