WASHINGTON — They were once seatmates in the spare limousine of the White House motorcade, traveling the globe together as part of the president’s inner circle.

Bonded by the miles they logged on the road and their unique access to power, Pete Souza, the former official White House photographer who took nearly two million photographs of former President Barack Obama, and Representative Ronny Jackson of Texas, the former White House physician who was elected to Congress as a Republican in 2020, were once close friends.

Now, they are the most public of enemies on social media, where Mr. Jackson routinely hurls insults and unsubstantiated claims of cognitive decline at President Biden and Mr. Souza responds with bitingly personal, sometimes salacious takedowns of the congressman’s character. He often begins them tauntingly with, “Hey Ronny.”

Their break is a particularly vivid and public example of how allegiance or opposition to former President Donald J. Trump has driven more Americans into partisan corners, sometimes transforming personal relationships in the process.

Mr. Jackson, once a little-known doctor on the White House medical staff, has morphed into a Trump-loving, MAGA Republican, one who became famous for praising Mr. Trump’s “great genes” while delivering the results of his physical. These days, he routinely alleges, without evidence, that Mr. Biden is senile and unfit to hold office. The transformation has left former Obama officials like Mr. Souza shocked and disgusted.

Mr. Souza is a former journalist who covered presidents before becoming Mr. Obama’s official White House staff photographer, and a key shaper of the 44th president’s public image. After leaving the White House, Mr. Souza became something of an activist and a microcelebrity among liberals, thanks in part to his social media feeds, where he used flattering photographs of Mr. Obama to troll Mr. Trump, ultimately compiling them into a book, entitled “Shade: A Tale of Two Presidents.”

Now, Mr. Souza uses his popular Twitter feed, which has more than 233,000 followers, almost exclusively to ridiculing and reproaching Mr. Jackson, who represents Texas’ 13th Congressional District, one of the most conservative in the country.

He accuses the congressman of spreading disinformation, getting his facts wrong about who is responsible for rising gas prices and rarely visiting his district when he’s not campaigning. But any of Mr. Jackson’s political foes could do that.

More notably, Mr. Souza has dug up photographs from his personal collection and posted pictures of Mr. Jackson asleep in hallways, accusing him of “being hungover while the on-duty doctor for the President of the United States.”

He also broadcasts lewd stories that he says Mr. Jackson shared with him in the past, including one eyebrow-raising anecdote involving the shaving of his nether regions.

“Hey Ronny,” he tweeted in February. “Remember when you told me the Saudis offered you $1 million annually to be the King’s doctor? But your wife didn’t want to live in the desert. (Others who were in the spare limo with us overheard this too.) Makes me wonder why you’re so un-American.”

When Mr. Jackson claimed that Mr. Biden is the worst president in his lifetime, Mr. Souza logged onto Twitter to correct the record.

“I remember when you told me how much you admired and respected Joe Biden,” he wrote in December. “Then you had that mind-altering cocktail: Trump kool aid mixed with alcohol.”

Mr. Jackson said he was aware of Mr. Souza’s tweets, but did not spend time dwelling on them.

“It seems pretty juvenile,” he said in an interview. “He seems to be totally consumed with me. We really were good friends and I have never said a single negative thing about him.”

Mr. Jackson insisted that Mr. Souza’s allegations did not get under his skin.

“It’s background noise,” he said.

Mr. Souza’s strategy is not popular among many of his former colleagues, some of whom said they shared Mr. Souza’s outrage over Mr. Jackson’s political shift, but preferred to keep quiet about it.

Mr. Jackson, a freshman congressman in the minority, does not have a large profile on Capitol Hill. He left the West Wing in 2018 after rising from Mr. Trump’s physician to his unlikely pick to lead the Department of Veterans Affairs.

He was forced to withdraw his name from consideration amid allegations related to his professional conduct. Instead, he ran for Congress, and with the help of Mr. Trump’s own former campaign managers, Bill Stepien and Justin R. Clark, won his seat in a crowded primary by emphasizing his close relationship to Mr. Trump.

Despite his junior status in Congress, Mr. Jackson is now a regular on Fox News, where throughout the pandemic he has contradicted guidance from the Centers for Disease Control and Prevention and stated that wearing a mask to prevent the spread of the coronavirus should be a personal choice.

Mostly, though, he has tried to distinguish himself by beating the drum about Mr. Biden’s mental state, claiming in Trumpian terms that “something’s going on here,” demanding the president take a cognitive test and insinuating that he has dementia.

It has been a depressing turn of events for many former Obama White House officials, who said they had always assumed Mr. Jackson was a Republican but had never regarded him as a partisan, much less a ruthless one. They liked Mr. Jackson and trusted him because he was gracious, even going out of his way to help relatives of staff members, several said, speaking on the condition of anonymity to describe internal interactions. (One former official recalled Mr. Jackson personally checking in on the father of a midlevel staff member who had suffered a heart attack.)

His unsubstantiated claims about Mr. Biden’s mental acuity have also raised questions among medical ethics experts.

“This kind of thing is irresponsible and is unethical when done to score political points rather than to help a patient or to protect the public from imminent threat,” said R. Alta Charo, a professor of law and bioethics at the University of Wisconsin-Madison.

The psychiatric community has counseled against offering diagnoses of public figures without access to a patient’s medical history, personal interviews and physical data. Still, there is little recourse for those who do so.

“Many states are known to have very weak medical boards that do not discipline as much as you would think,” Ms. Charo said.

And Mr. Jackson, a retired Navy rear admiral, said that he let his medical license expire last year because he did not have time to see patients, although he still references himself as “Dr. Ronny Jackson” on his campaign materials and his official congressional website.

He said he was not violating any confidentiality with a former patient because he never served as Mr. Biden’s physician.

“I saw him around in the halls, but I wasn’t responsible for his medical care,” Mr. Jackson said.

For his part, Mr. Souza said he does not plan to stop anytime soon.

“As long as Ronny continues to tweet disinformation and lies about President Biden and/or Covid, I will continue to respond with the truth, no matter how uncomfortable that may be,” he said in an email.