In 2016, state SenatorAdriano Espaillat ran for U.S. House of Representatives in the race to replace retiring long-serving Congressman Charlie Rangel of Harlem. Upon winning that election, Espaillat's state Senate seat became open. While Alcantara did not declare her candidacy until after Espaillat won his Congressional primary in June 2016, she entered the race to succeed him when it became clear no other notable Dominican candidate would emerge. Facing Bloomberg administration alumnus Micah Lasher and former New York City Council member Robert Jackson, Alcantara narrowly won the September 2016 Democratic primary with a plurality of 31% of the vote; the race was considered one of the elections to watch in the 2016 state primaries, and emerged as one of the closest that cycle. Alcantara easily won the general election in the heavily Democratic district with over 85% of the vote. Upon winning her seat, Alcantara announced that she would join the Independent Democratic Conference, a group of state senate Democrats that allied with the senate Republicans, allowing Republicans to control the chamber. This did not come as a surprise, as she owed Independent Democratic Leader Jeffrey D. Klein for his assistance in her primary election campaign. After joining the majority coalition, the freshman Senator was named Chair of the Labor Committee. According to The New York Times, Alcantara "condemned" a bill by Democrats designed to stop deportations in New York State. Alcantara proposed a bill that raised the limit for discretionary contracts for minorities and women from $20,000 to $150,000 that passed. In April 2018, Alcantara and her IDC colleagues rejoined the Senate Democratic Conference. Subsequently, the Republican conference stripped Alcantara of her position as Chair of the Labor Committee. In the September 2018 Democratic primary election, Alcantara was challenged again by Jackson as well as by Thomas Leon and Tirso Santiago Pina. This time, Jackson won the four-way primary in a landslide with 56% of the vote to Alcantara's 38%. Alcantara's defeat was attributed to long-simmering anger with the former members of the IDC. As part of New York State'selectoral fusion laws allowing candidates to run on multiple ballot lines in one election, Alcantara still appeared in the November 6, 2018 general election as the third-party Independence Party of New York candidate; however, Alcantara conceded to Jackson and pledged to work with him.