Republican U.S. presidential candidate Donald Trump on Sunday levelled an attack against Taylor Swift, declaring his "hate" for her just days after the pop mega-star endorsed his Democratic opponent, Kamala Harris.

Trump wrote in an all-caps posting on his Truth Social media account, "I HATE TAYLOR SWIFT!"

Immediately following last week's Trump-Harris debate, Swift informed her 284 million Instagram followers that she planned to vote for Harris, currently the U.S. vice president, "because she fights for the rights and causes I believe need a warrior to champion them."

Trump, who has 7.71 million followers on Truth Social, initially had dismissed Swift's endorsement of Harris by saying he was "not a Taylor fan."

But since then, with public opinion polls showing Harris gaining significant ground on Trump in what is expected to be a close Nov. 5 presidential election, the former president has ratcheted up his rhetoric against one of the most successful recording artists in history.

Swift's backing of Harris has drawn over 9 million "likes" to her Instagram post, fueling speculation that it could boost the Democrat's chances of winning the presidential election.

After the Trump post, Harris' vice presidential running mate Tim Walz wrote on X: "Swifties: With your help we're gonna defeat the smallest man who ever lived."

One of Swift's songs is titled "The Smallest Man Who Ever Lived."

Trump's rage against Swift brought a torrent of supporters to proclaim on X, "I LOVE TAYLOR SWIFT," while others came to Trump's defence by attacking the administration of Democratic President Joe Biden.

Trump and Swift have been trading barbs for years.

After she supported Democratic candidates in the 2018 elections, Trump said in response to reporters' questions at the White House: "Let's say that I like Taylor's music about 25% less now."