Rudy Giuliani is being treated in hospital after testing positive for Covid-19.
President Donald Trump’s personal lawyer is the latest person close to the president to be infected.
Since November, Rudy Giuliani, who has led the Trump campaign’s legal challenges to the election results, has been on a cross-country tour in an effort to convince state governments to overturn the vote.
Like other Trump officials, he has been criticized for shunning face masks.
President Trump, who got the virus in October, announced the diagnosis in a tweet, writing: “Get better soon Rudy, we will carry on!”
The former New York mayor was admitted to the Medstar Georgetown University Hospital in Washington DC on December 6.
The news came after he had visited Arizona, Georgia and Michigan all in the past week – where he spoke to government officials while not wearing masks.
Following news of Rudy Giuliani’s diagnosis, the Arizona legislature announced sudden plans to shut down for one week. Several Republican lawmakers there had spent over 10 hours with him last week discussing election results.
Following Rudy Giuliani’s visit to Phoenix, Arizona, the state’s Republican party tweeted a photo of him with other mask-less state lawmakers.
In a tweet, the 76-year-old thanked well-wishers for their messages, and said he was “recovering quickly”.
He wrote: “Thank you to all my friends and followers for all the prayers and kind wishes. I’m getting great care and feeling good. Recovering quickly and keeping up with everything.”
Rudy Giuliani’s son, who works at the White House and tested positive for the virus last month, tweeted that his father was “resting, getting great care and feeling well”.
He wrote: “My Dad @RudyGiuliani is resting, getting great care and feeling well. Thank you to all the friends who have reached out concerned about his well being.”
On December 6, Dr Deborah Birx, the White House coronavirus task force coordinator, criticized the Trump administration for flouting guidelines and peddling “myths” about the pandemic.
She told NBC: “I hear community members parroting back those situations, parroting back that masks don’t work, parroting back that we should work towards herd immunity.”
“This is the worst event that this country will face,” she said.
Dozens of people in President Trump’s orbit are said to have tested positive for Covid-19 since October.
Boris Epshteyn, another Trump adviser, tested positive shortly after appearing alongside Rudy Giuliani at a news conference on November 25.
Others include the president’s chief of staff Mark Meadows and press secretary Kayleigh McEnany, along with First Lady Melania Trump and sons Donald Jr. and Baron.