The 28-year-old says events in "the past two years" have meant she's no longer reluctant to share her views.
The singer wrote on Instagram: "I always will cast my vote based on which candidate will protect and fight for the human rights I believe we all deserve."
The elections are on 6 November.
Taylor, who in June gave a speech in Chicago offering "love and respect" to LGBT people who haven't yet come out, will be voting in historically Republican state Tennessee.
Media captionDonald Trump on Taylor Swift: "Lets just say I like her music about 25% less now"
"I believe in the fight for LGBT rights, and that any form of discrimination based on sexual orientation or gender is wrong," she published to her 112m followers.
"I believe that the systemic racism we still see in this country towards people of colour is terrifying, sickening and prevalent."
What are the US mid-terms?
"As much as I have in the past and would like to continue voting for women in office, I cannot support Marsha Blackburn. Her voting record in Congress appals and terrifies me," she wrote.
"She voted against equal pay for women. She voted against the Reauthorisation of the Violence Against Women Act, which attempts to protect women from domestic violence, stalking, and date rape."
One of the candidates endorsed by Taylor, former Tennessee Governor Phil Bredesen, backed Brett Kavanaugh's controversial successful Supreme Court nomination.
Tennessee usually sides with President Donald Trump's Republican party, but recent polls suggest the result there could be close.
The Instagram post has had more than a million likes, while users on Twitter used Taylor's letter as an opportunity to speak on one of her historical foes - Ye (formerly known as Kanye) West.
They remembered Ye's pledge to run for president.
Taylor's comments came the day after Kanye once again deleted his social media profiles.