NEW YORK -- President-elect Donald Trump is claiming, without evidence, that millions of people voted illegally in the election he won, issuing the baseless claim as part of his angry response to a recount effort led by the Green Party and joined by Hillary Clinton's campaign.

"I won the popular vote if you deduct the millions of people who voted illegally," Trump tweeted Sunday. He later alleged "serious voter fraud in Virginia, New Hampshire and California."

Trump's transition team did not provide any evidence to back up the president-elect's assertions of fraud in the November election. They pointed only to past charges of irregularities in voter registration.

Sen. James Lankford, R-Okla., said Monday he had "not seen any voter irregularity in the millions."

"I don't know what he was talking about on that one," Lankford said of Trump on CNN's "New Day."

Indeed, there has been no evidence of widespread tampering or hacking that would change the results of the presidential contest between Trump and Clinton. The Democrat's team said it had been looking for abnormalities and found nothing that would alter the results.

Still, Clinton's campaign was joining a recount led by Green Party candidate Jill Stein in up to three states. The Wisconsin Elections Commission voted unanimously Monday to approve a recount that could begin Thursday, after payment is received from Stein and an independent candidate also requesting a recount. Recounts are possible in Pennsylvania and Michigan as well.

"We intend to participate in order to ensure the process proceeds in a manner that is fair to all sides," Clinton campaign attorney Marc Elias said.

Trump narrowly won Wisconsin and Pennsylvania and, as of last Wednesday, held a lead of almost 11,000 votes in Michigan, with the results awaiting state certification Monday. All three would need to flip to Clinton to upend the Republican's victory, and Clinton's team says Trump has a larger edge in all three states than has ever been overcome in a presidential recount.

After spending the Thanksgiving holiday in Florida, Trump was back in New York for meetings with potential Cabinet nominees. His team was divided over his choices for secretary of state, particularly the prospect that Trump could tap 2012 GOP nominee Mitt Romney for the prominent post.

Romney is scheduled to hold his second in-person meeting with Trump on Tuesday. But the president-elect is also looking at other options, meeting Monday with retired Gen. David Petraeus and on Tuesday with Tennessee Sen. Bob Corker, the chairman of the Senate Foreign Relations Committee.

In an unusual public airing of internal machinations, Trump senior adviser Kellyanne Conway warned Sunday that the president-elect's supporters would feel "betrayed" if he tapped Romney as secretary of state. Romney denounced Trump in scathing terms during the campaign, prompting Trump to call him a "choker" who "walks like a penguin."

The spectacle of close aides who speak frequently with Trump in private being so explicit about their personal opinions in public raised the possibility that Conway was acting at Trump's behest.

People involved in the transition process said Trump's decision on his secretary of state did not appear to be imminent. Former New York City Mayor Rudy Giuliani was initially seen as the favourite for the diplomatic post, but Trump is said to have grown irritated by questions about Giuliani's international business ties, as well as the mayor's public campaigning for the job.

Even with major administration decisions looming, Trump seems preoccupied by the prospect of a recount.

"Hillary Clinton conceded the election when she called me just prior to the victory speech and after the results were in," He tweeted Sunday. "Nothing will change."

He quoted from Clinton's concession speech -- "We owe him an open mind and the chance to lead" -- and he concluded: "So much time and money will be spent - same result! Sad."

On NBC's "Meet the Press," Conway said Stein, "the Hillary people" and others supporting recounts have to decide whether they are going to back a peaceful transition "or if they're going to be a bunch of crybabies and sore losers about an election that they can't turn around."

Clinton's lawyer said her team has been combing through the results since the election in search of anomalies that would suggest hacking by Russians or others and found "no actionable evidence." But "we feel it is important, on principle, to ensure our campaign is legally represented in any court proceedings and represented on the ground in order to monitor the recount process itself," he said.

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Pace reported from Washington. AP writers Laurie Kellman, Steve Peoples and Cal Woodward contributed to this report