Defending Elections against Trolls from Enemy Regimes Act or DETER Act This bill amends the Immigration and Nationality Act to make an alien who is seeking to engage in, or has engaged in, improper interference in a U.S. election inadmissible. Improper interference in a U.S. election is conduct by an alien that: (1) violates federal criminal, voting rights, or campaign finance law, or is under the direction of a foreign government; and (2) interferes with any general or primary federal, state, or local election or caucus, including a candidate's campaign or a ballot measure.