Donald Trump has hit back at Taylor Swift after the pop star broke her political silence and revealed she was backing two Democratic candidates, saying he now likes her music "25 per cent less".

Swift, who has been criticised in the past for refusing to disclose her political opinions, used Instagram to announce her support for Phil Bredesen for the Senate and Jim Cooper for House of Representatives, as she prepares to vote in Tennessee in the midterm elections in November.

The 28-year-old said she had chosen to speak out now "due to several events in my life and in the world in the past two years".

Mr Trump defended the Republican candidate in Tennessee, Marsha Blackburn, and said he now liked Swift's music "25 per cent less".

Speaking outside the White House, Mr Trump told reporters: "Marsha Blackburn is doing a very good job in Tennessee. She's leading now, substantially, as she should. She's a tremendous woman, I am sure Taylor Swift does not know anything about her and let's say that I like Taylor's music 25 per cent less now."

In her Instagram post, Swift said she felt compelled to speak out because she believes in "the fight for LGBTQ rights" and that "any form of discrimination based on sexual orientation or gender is WRONG".

She added: "I believe that the systemic racism we still see in this country towards people of colour is terrifying, sickening and prevalent."

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Swift went on to say that she will not vote for Marsha Blackburn, despite her desire to see women in office.

"I cannot vote for someone who will not be willing to fight for dignity for ALL Americans, no matter their skin colour, gender or who they love," she said.

Swift added: "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 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.

"She believes businesses have a right to refuse service to gay couples. She also believes they should not have the right to marry. These are not MY Tennessee values."

Swift, who asked her 112 million followers to "educate" themselves on the candidates in their states and to choose the one who "who most closely represents your values".

She added: "For a lot of us, we may never find a candidate or party with whom we agree 100% on every issue, but we have to vote anyway. So many intelligent, thoughtful, self-possessed people have turned 18 in the past two years and now have the right and privilege to make their vote count."

Swift lives in New York but spent her later years in Tennessee.

Mr Bredesen thanked Swift for her endorsement in a tweet, writing: "Thank you for the kind words taylorswift13. I'm honoured to have your support and that of so many Tennesseans who are ready to put aside the partisan shouting and get things done. We're ready for it."