The president-elect, who offered no evidence for his claims, earlier called the recount effort a “scam”, while senior adviser Kellyanne Conway called Green party candidate Jill Stein and Clinton “a bunch of crybabies and sore losers”.
Marc Elias, general counsel for the Clinton campaign, wrote on Saturday that the campaign would support Stein’s effort in Wisconsin, where a recount will take place. Stein is also pushing for recounts in Pennsylvania and Michigan and has raised more than $6m online to fund such efforts.
The decision put the Clinton camp at odds with the Obama White House, which has expressed confidence in election results.
On Saturday, Trump attacked Stein, using Twitter to say: “The Green Party scam to fill up their coffers by asking for impossible recounts is now being joined by the badly defeated [and] demoralized Dems.”
On Sunday morning, the president-elect fired off a volley of tweets, starting: “Hillary Clinton conceded the election when she called me just prior to the victory speech and after the results were in. Nothing will change.”
The president-elect then drew attention to a debate remark by Clinton after Trump refused to commit to accepting the election result, quoting her as saying: “That is horrifying. That is not the way our democracy works.
“Been around for 240 years. We’ve had free and fair elections. We’ve accepted the outcomes when we may not have liked them, and that is what must be expected of anyone standing on a during a general election.
“I, for one, am appalled that somebody that is the nominee of one of our two major parties would take that kind of position.”
Trump was due back in New York on Sunday after spending Thanksgiving at his Mar-a-Lago resort in Florida, where one report said he had been asking visitors who should be his secretary of state.
In the afternoon, around the time of his scheduled departure for Manhattan, he used Twitter to say: “In addition to winning the electoral college in a landslide, I won the popular vote if you deduct the millions of people who voted illegally.”