President-elect Donald Trump, his wife, Melania Trump, and son Barron Trump board his plane headed back to New York after spending the holiday weekend in Mar-a-Lago in Palm Beach, Fla., on Sunday. (Jabin Botsford/The Washington Post)

President-elect Donald Trump spent Sunday ridiculing Hillary Clinton’s presidential campaign for joining a recount effort in Wisconsin, ending his day on Twitter by parroting a widely debunked conspiracy theory that her campaign benefited from massive voter fraud.

As his senior advisers engaged in an escalating feud over who the next secretary of state should be, Trump focused publicly on Clinton’s tally of 64 million votes — more than 2 million beyond what he garnered — by suggesting without evidence that millions of people illegally voted in the election.

“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,” Trump tweeted late Sunday, one of more than 10 tweets on the recount issue.

That accusation — spread by conspiracy sites such as ­Infowars.com and discredited by fact-checking organizations — gained traction among some far-right conservatives disappointed that Trump lost the popular vote.

[Fact Checker: Donald Trump’s bogus claim that millions of people voted illegally for Hillary Clinton]

Republicans and Democrats react to the announcement that Hillary Clinton’s campaign plans to join a vote recount in Wisconsin initiated by former Green Party presidential candidate Jill Stein. (Bastien Inzaurralde/The Washington Post)

But Trump’s embrace of the claim created even more instability around the election results from both ends of the political spectrum, with Green Party candidate Jill Stein leading calls among liberal activists for recounts in key battleground states to make sure voting fraud did not give the election to Trump.

The charges and countercharges could present a highly toxic issue for the Justice Department, including Sen. Jeff Sessions ­(R-Ala.), Trump’s nominee to be attorney general, whose tenure, if confirmed, comes amid a long-running battle over renewal of the Voting Rights Act. The two parties are already locked in fights over ballot access, with Republicans advocating ID requirements and other limitations that Democrats say are aimed at suppressing the votes of minorities and others more likely to vote Democratic.

Trump spent the Thanksgiving weekend out of the public spotlight, holed up inside his Mar-a-Lago estate in Florida and continuing his private deliberations about building the next administration. Kellyanne Conway, his former campaign manager and now senior adviser to the transition team, said on NBC’s “Meet the Press” that Trump and President Obama spoke by phone for about 45 minutes on Saturday, the latest in a series of conversations the two men have had since the bitter campaign concluded.

Conway also led a remarkable rear-guard attack on 2012 GOP nominee Mitt Romney as a possible secretary of state, arguing that the former Massachusetts governor was disloyal to Trump and should not be considered for the post. The remarks highlighted a fierce behind-the-scenes battle within the Trump camp over whether Romney should be offered the job as a healing gesture with the establishment or instead whether a loyalist such as former New York mayor Rudolph W. Giuliani should get the nod.

Rather than referee that dispute, however, Trump’s only public proclamations came over Twitter — his favorite means of communication — beginning at 7:19 a.m. Sunday, when he lambasted the Clinton campaign for agreeing to take part in a recount of votes in Wisconsin spearheaded by Stein.

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

Trump then composed five tweets quoting parts of Clinton’s responses at the third presidential debate, during which she blasted Trump for telling moderator Chris Wallace that he would “keep you in suspense” rather than outright promise to accept the election results.

At the debate, Clinton had called Trump’s answer “horrifying” and “a direct threat to our democracy.”

On Saturday, Clinton campaign counsel Marc Elias said Clinton’s advisers had been monitoring voting patterns in critical states to see whether there were any signs of foul play, particularly in the wake of hacking of emails at the Democratic National Committee and of senior campaign officials earlier this year. Elias wrote that no evidence of wrongdoing had emerged in their preliminary review.

But once Stein’s campaign formally started the recount process in Wisconsin, Elias said Clinton’s campaign would join the effort even if her top advisers did not expect the outcome to change in any of the Midwestern states that tipped the electoral college balance to Trump.

“We had not planned to exercise this option ourselves, but now that a recount has been initiated in Wisconsin, we intend to participate in order to ensure the process proceeds in a manner that is fair to all sides,” he said.

Clinton’s campaign plans to do the same if Stein, who has raised more than $5 million to help fund the recounts, follows through on plans to ask for the same process in Michigan and Pennsylvania.

[Clinton campaign will participate in Wisconsin recount, with an eye on ‘outside interference,’ lawyer says]

The total margin in those three states was 107,000 votes out of nearly 13.8 million cast, with a combined 46 electoral votes among them — enough to give the race to Clinton had she won those three battlegrounds.

Clearly irritated by losing the popular vote, Trump suggested that the math to getting to the 270-vote margin in the electoral college made him come up short. “It would have been much easier for me to win the so-called popular vote than the Electoral College in that I would only campaign in 3 or 4 states instead of the 15 states that I visited,” he tweeted.

Meanwhile, the extraordinary feud over his pick for secretary of state hit a fevered pitch when Conway took to three Sunday television shows to attack Romney’s credentials to be the nation’s top diplomat. Conway is part of a cadre of Trump confidants who are waging an unusual public battle against Romney to influence the ­president-elect’s decision.

Romney’s supporters have largely counseled Trump privately, while his opponents — Conway, former House speaker Newt Gingrich and a handful of other Trump loyalists — have gone public in recent days to stoke a revolt.

Those opposed to Romney have been advocating Trump to reward Giuliani with the secretary of state position, stressing his loyalty vs. Romney’s refusal to publicly say whom he voted for in the presidential race.

“I don’t think a cost of admission for party unity has to be the secretary of state position,” Conway said on ABC’s “This Week,” suggesting Romney went too far in questioning Trump’s character and intellect during the presidential race. “There was the ‘Never Trump’ movement and then there was Governor Mitt Romney. He went out of his way to hurt Donald Trump.”

The Conway offensive, which began Thursday on Twitter and continued in her three appearances Sunday, led the echo chamber of opposition to the former 2012 Republican presidential nominee. Gingrich said most Trump supporters could not countenance Romney’s selection for any high-profile posting in Trump’s new administration.

“I think we would be enormously disappointed if he brought Mitt Romney into any position of authority,” Gingrich said on Fox News.

Others inside Trump’s orbit, particularly Vice President-elect Mike Pence, are viewed as Romney supporters, believing he has a steady hand that would benefit Trump in the turbulent world of diplomacy, according to people familiar with the dispute who spoke on the condition of anonymity to discuss internal deliberations. The incoming White House chief of staff, Reince Priebus, also has a long-standing relationship with Romney since he served as Republican National Committee chairman during Romney’s unsuccessful 2012 presidential bid.

Infighting over such a prominent position is not unusual. Four years ago, President Obama faced internal division over his choice to succeed Clinton as secretary of state, with one possibility being Susan E. Rice, who had been a longtime Obama confidant. Senate Democrats preferred their colleague, John F. Kerry, and eventually Obama made Rice his national security adviser and gave Kerry the State Department.

However, that internal Democratic feud played out in the traditional Washington manner — behind closed doors and in anonymous background quotes to news outlets.

Today’s Republican battle has been carried out in an unusually public fashion that left some old Washington hands stunned and wondering whether this would be the new normal over the next four years.

“I have never, ever, seen any aide to a POTUS or PEOTUS publicly try and box the boss in like this. Extraordinary,” David Axelrod, a senior adviser to Obama’s two presidential campaigns, wrote on Twitter, using the acronyms for president and president-elect.

Axelrod later questioned whether Conway was assigned the job of trashing Romney, or else it was an “unbelievable breach” for a staffer to do so while Trump was still making his choice.

Philip Rucker, Amy B Wang and Kristine Guerra contributed to this report.