Some Democratic strategists have criticized her “ground game” in Pennsylvania, which is considered the most important state.
Joe Biden, who will leave the White House in January after just one term as president, triumphed in Pennsylvania in 2020.
Virginia is a reliably Democratic voting area and was won by Biden in 2020, although some Trump strategists suggest that may “play into play” in this election.
Nevada has also been a belligerent state over the last century, and was most recently won by Biden, although recent polls have put Trump in the lead.
Downing Street insisted Sir Keir would be ready to work with whoever succeeds Biden and becomes the 47th president of the United States.
During a September visit to New York, he had a two-hour dinner with Trump after the former president praised the “popular” prime minister.
Sir Keir requested the meeting in a bid to build ties with the Republican candidate.
Although he had intended to meet Ms Harris, Sir Keir said “diary challenges” meant this was not possible.
Foreign Secretary David Lammy has been criticized for previous comments about Trump, which included the claim that he was a “sociopath with neo-Nazi sympathies.”
In September, Sir Keir refused to endorse Home Secretary Angela Eagle after she suggested Trump had emboldened racists in Britain after the riots.
He insisted he had made it “absolutely clear” that full responsibility for the riots that gripped Britain after the Southport killings lay with those who took part.