A day after the U.S. Supreme Court denied Texas' challenge to Biden's 2020 presidential election victory, Trump supporters gathered in Washington, D.C. to protest alleged voter fraud.

The "March for Trump" event, which started at noon on Saturday, includes a march to the U.S. Supreme Court to "demand transparency and protect election integrity," according to promotional materials. 

Former Trump administration national security adviser Mike Flynn was among those in attendance. 

Trump tweeted Saturday morning that he just found out about the rally and said "I'll be seeing them."

On Friday, the Supreme Court rejected a lawsuit brought by the state of Texas and Republican allies of Trump to throw out millions of votes in four states and overturn Biden's victory. In dismissing the case, the court said Texas had no "cognizable interest" in how Pennsylvania, Michigan, Wisconsin and Georgia conduct their own elections.

The rally also comes as members of the Electoral College will meet Monday to formally cast their votes for president.

FDA Commissioner Stephen Hahn on Saturday pushed back on reports that he was threatened with firing.

Sources familiar with the matter told ABC News that in a Friday phone call, White House chief of staff Mark Meadows suggested to Hahn that his job could be on the line if his agency didn’t authorize emergency use of the Pfizer-BioNTech vaccine by the end of the day.

The Pfizer-BioNTech vaccine was authorized by the FDA late Friday.

“Representations in the press that I was threatened to be fired if we didn’t get it done by a certain date is inaccurate,” Hahn told reporters on a Saturday morning call.

Hahn said Saturday that the vaccine was authorized late Friday because science and data determined it was ready, not because of “any other external pressure” and that he would “absolutely” take the vaccine.

-ABC News' Emily Shapiro, Katherine Faulders, John Santucci and Anne Flaherty

Biden is headed to Philadelphia today for a follow-up foot scan, two weeks after sustaining small hairline fractures in his mid-foot.

“The president-elect will have a follow-up appointment at the radiology department of the Pennsylvania Hospital,” according to the Office of President-elect Biden.

Biden’s doctor, Dr. Kevin O’Connor, provided more background on the scan in a statement. 

“Consistent with our original plan, President-elect Biden will receive routine 2-week post-injury imaging today. This will be performed with a special CT scanner which is able to obtain a “weight-bearing” image. Now that the initial discomfort and swelling are decreased, it is important to observe the structures within the midfoot under the actual pressure of standing. This is the best way to assure ankle and foot stability,” the statement said.

After the scan, O’Connor said: “Weight-bearing CT results were very encouraging. The small fracture in the intermediate cuneiform is barely detectable and the small fracture in the lateral cuneiform is healing as expected. No more extensive injury was identified.”

Biden was spotted as he was leaving the appointment, where he waved and gave a thumbs up to the press corps.

There was a group of people gathered who cheered for Biden. Biden clearly noticed that one man was not wearing a mask and signaled as if to suggest he put one on. The older man just began to blow kisses as the president-elect, according to the TV pooler.

Biden then headed back to Wilmington.

-ABC News' Molly Nagle

The U.S. Supreme Court late Friday officially put to rest a brazen eleventh-hour attempt by the state of Texas and Republican allies of President Donald Trump to throw out millions of votes in four states and overturn Democrat Joe Biden’s victory in the 2020 presidential election.

In an unsigned, single-page order, the court rejected a lawsuit brought by Texas, citing a lack of standing under Article III of the Constitution. In dismissing the case, the court said Texas had no "cognizable interest" in how Pennsylvania, Michigan, Wisconsin and Georgia conduct their own elections.

The decision, coming just three days before the Electoral College meets to finalize the presidential selection, shut down what Trump had called "perhaps the most important case in history" without the justices getting into the merits of arguments on either side.

For more on the critical ruling, read ABC News' full story.

-ABC News' Devin Dwyer