Her race against Republican Martha McSally was so tight that her victory was not decided until Monday, after a slow count of postal ballots gave her an insurmountable lead.
McSally's attacks on Sinema reached back more than 15 years to when Sinema was a Green Party spokeswoman and liberal activist. Her victory is the first time a Democrat has won a U.S. Senate seat in Arizona in 30 years, and means Democrats will have at least 47 seats in the Senate-as results from Florida and MS remain uncalled.
Democrat Rep. Kyrsten Sinema has officially won Arizona's open Senate seat, nearly a week after the midterm congressional elections, reducing Republicans' net gain in the chamber to a single seat. Results in Florida and MS are still outstanding.
Ms Sinema, 42, is also openly bisexual, and has spoken about facing bullying as a child because of her sexuality.
Despite its image as a staunch Republican bastion, Arizona is attracting younger, educated voters from elsewhere in the United States.
U.S. Rep. Kyrsten Sinema was declared victor of the drawn-out Senate race.
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Sinema prevailed while the Democratic candidate for governor, David Garcia, ran as an avowed progressive and got trounced by Republican incumbent Doug Ducey. McSally also campaigned on her military record and support for the armed forces. At a debate, she called Sinema a traitor to America.
McCain's seat is temporarily being held by Republican Jon Kyl, who has said that he will hold it until the end of this year. "Second, while the National Republican Senatorial Committee's conduct in casting doubt on the vote-counting process was reprehensible, Ducey, Cindy McCain and McSally behaved appropriately, refusing to join in the anti-democratic process of delegitimizing elections that don't go their way".
She positioned herself as having voted to support Mr Trump's agenda 60% of the time.
In 2012, Baldwin made history by becoming the first openly gay person elected to Senate, in addition to being the first woman elected to represent Wisconsin in Congress.
McSally hammered Sinema over her former liberal stances and claimed she was pretending to be a centrist.
She also underscored the fact that she had not voted for Nancy Pelosi for House Speaker. About 2.1 million Latinos live in Arizona, about a third of the state's population; while Mr Trump narrowly carried the state in the 2016 election, many Latinos in the state have expressed displeasure with the President. Mr Rohrabacher was one of most vocal supporters of Russian Federation and its President Vladimir Putin in Congress.