Donald Trump Jr. campaigns for his father in Gilbert, Ariz., on Nov. 4. (Matt York/AP)

Donald Trump Jr., the eldest of the president’s five children, played a familiar role on his father’s behalf last July, vociferously dismissing as “disgusting” and “phony” any suggestion that the Russians were attempting to aid his father’s presidential campaign.

The month before that CNN interview, however, Trump Jr. himself had convened a meeting in Trump Tower with a Russian lawyer who promised to provide damaging information about Democratic challenger Hillary Clinton.

Trump Jr.’s new acknowledgment that the meeting occurred and his shifting explanations of what it entailed have thrust him into the spotlight of the biggest controversy surrounding his father’s presidency: investigations of possible collusion during last year’s election between the Trump campaign and Russia.

The centrality of his role was underscored Monday afternoon with word that Trump Jr., 39, had retained a criminal defense lawyer, New York-based Alan S. Futerfas, whose past clients have included embattled politicians, computer hackers and alleged organized-crime associates.

The stakes grew higher still with a New York Times report Monday night that Trump Jr. was informed in an email that the promised material was part of a Russian government effort to aid his father’s candidacy.

(Jenny Starrs/The Washington Post)

Unlike his sister Ivanka Trump and her husband, Jared Kushner — both of whom have top White House jobs — Trump Jr. chose to stay behind in Manhattan after the election, taking control with his brother Eric Trump of the president’s business interests. He has been a relatively infrequent visitor to Washington, appearing only at select events, including the nomination of Judge Neil M. Gorsuch to the Supreme Court and the White House Easter Egg Roll.

But that has hardly diminished Trump Jr.’s fierce loyalty and outspoken advocacy for his father’s political interests. On talk radio and Twitter, he has become omnipresent — taking on Democrats, the media and anyone else perceived to be standing in his father’s way, often in terms at least as provocative as those of the president himself.

“He can say basically what everyone’s thinking but may feel constrained about saying because of their official positions,” said one adviser to President Trump who spoke on the condition of anonymity to talk more freely.

Over the weekend, as the president took flak for briefly turning over his chair at the Group of 20 summit in Hamburg to Ivanka Trump, Trump Jr. took aim at critics of his father and his sister.

“She is VERY smart & eloquent. You can belittle her all you want w your snark, but we all know 1 on 1 she way out of your league,” Trump Jr. said on Twitter in response to Republican consultant Ana Navarro, who had mocked his sister’s time in the chair.

Trump Jr. took to Twitter several times again Monday in his own defense.

Donald Trump Jr. arrives at Trump Tower in New York City on Jan. 18. (Stephanie Keith/Reuters)

He started the day on a sarcastic note, responding to the news, first reported by the New York Times, that he had arranged a meeting with a Russian lawyer with ties to the Kremlin claiming to have dirt on Clinton.

“Obviously I’m the first person on a campaign to ever take a meeting to hear info about an opponent,” Trump Jr. wrote, adding, “Went nowhere but had to listen.”

Later, he sought to rebut the notion that his explanation for the meeting had changed.

Trump Jr. initially said Saturday that the meeting was about an adoption program that the Kremlin had cut off in retaliation for a U.S. law that targeted Russian human rights abusers. But in a statement Sunday, Trump Jr. said an acquaintance asked him to meet with lawyer Natalia Veselnitskaya because she claimed to have information about Clinton.

“No inconsistency in statements, meeting ended up being primarily about adoptions,” Trump Jr. said. “In response to further Q’s I simply provided more details.”

Still later Monday, he responded to a report that the Senate Intelligence Committee wants to interview him, tweeting, “Happy to work with the committee to pass on what I know.”

Trump Jr. grew up in Trump Tower and graduated from his father’s alma mater, the Wharton School at the University of Pennsylvania, before rising through the ranks of the Trump Organization. He appeared as a boardroom adviser on “The Apprentice,” his father’s hit reality show on NBC.

Trump Jr. was introduced to his wife, Vanessa Haydon, by his father, and the couple were married at Trump’s Mar-a-Lago Club in Palm Beach, Fla.

But he has also carved out an identity independent of his father. Growing up, he spent summers hunting with his grandfather in what was then Czechoslovakia. He remains an avid hunter, as comfortable in the halls of a National Rifle Association convention as with the Manhattanites with whom his father surrounded himself in business.

During the campaign, Trump Jr. was frequently dispatched to gun-loving and flag-waving areas in red states, while Ivanka Trump was sent to woo suburbanites.

Barry Bennett, a Republican operative who advised Donald Trump during the general election campaign, said his eldest child was particularly effective because “he wasn’t worried about what his father’s campaign would mean to him or his brand.”

Trump Jr.’s speech at the Republican National Convention in Cleveland provided a breakout moment of sorts as he praised his father’s “unrelenting determination” and prompted speculation about a career ahead for him in politics — a notion he played down.

Besides political commentary, Trump Jr.’s Twitter feed has also featured windows into other aspects of his life. In February, he relayed that his wife had “dragged” him to the movie “Fifty Shades Darker” and he noted that he was “the only guy in an otherwise packed theater.”

“It’s two hours of my life I’ll never get back,” Trump Jr. wrote.

His father’s campaign was Trump Jr.’s first real foray into politics, and an adviser to President Trump on Monday characterized the meeting on June 9, 2016, with the Russian lawyer as a “rookie mistake.”

“It’s something that someone who’s never been around politics, particularly at the presidential level, might do but the rest of us would not,” said the adviser, who spoke on the condition of anonymity to more candidly discuss the episode.

The meeting was also attended by Kushner and the campaign’s chairman at the time, Paul Manafort. The adviser said it was particularly unwise to expose Manafort to a meeting with someone whom Trump Jr. claims not to have known.

Another Trump adviser described Trump Jr.’s actions as “well-meaning but naive.”

“You have to remember, the campaign was very unsophisticated at that point,” said the second adviser, who spoke on the condition of anonymity to talk more candidly. “It wasn’t that surprising that someone would be able to get a meeting. There wasn’t the kind of vetting going on that should have been.”

The meeting was the latest involving Russians that Trump associates initially failed to disclose. It also stood out because of the involvement of key players in the president’s inner circle.

As an executive in his father’s company, Trump Jr. was active in pursuing Trump Organization business prospects in Russia. He traveled to Moscow along with Ivanka Trump in 2006 and also helped pitch Trump-branded real estate to Russians.

“Russians make up a pretty disproportionate cross-section of a lot of our assets,” Trump Jr. told a real estate conference in 2008, according to a trade publication. “We see a lot of money pouring in from Russia.”

In the speech, he said he had traveled to Russia half a dozen times in the previous 18 months.

In October 2016, just weeks before his father’s election, Trump Jr. delivered a paid speech in Paris to a group whose leaders are close to Russia.

The speech was in front of the Center of Political and Foreign Affairs, an advocacy group founded by a French businessman and his partner who are known in France to work closely with Russian business interests.

The partner, Randa Kassis, told the Wall Street Journal in November that shortly after the election, she traveled to Moscow and held the dinner with Trump Jr. and an official in the Russian foreign ministry.

A spokeswoman for the president’s son has previously responded to questions about the event by noting that Trump Jr. has been giving paid speeches for over a decade, discussing a “range of topics.”

In an interview with the New York Times in March, Trump Jr. denied participating in any campaign-related meetings with Russian nationals.

“Did I meet with people that were Russian? I’m sure, I’m sure I did,” he said. “But none that were set up. None that I can think of at the moment. And certainly none that I was representing the campaign in any way, shape or form.”

Trump Jr.’s responses to news reports of recent days about his meeting with Veselnitskaya were notable for his protection of those around him.

By Sunday, Trump Jr. was acknowledging the meeting with Veselnitskaya — but also said that his father knew nothing about it and that he had asked Kushner and Manafort to attend without telling them what it was about.

In a tweetstorm of his own on Monday morning, President Trump went on the attack against a range of targets — from Chelsea Clinton to former FBI director James B. Comey — but made no mention of his son’s plight.

On Saturday, as the story was still evolving, Trump Jr. showed some of the combativeness he has often exhibited on Twitter.

“I love being attacked by pundits who somehow make a living in politics but haven’t been right about anything in 2 years,” he wrote. “So out of touch!”

Tom Hamburger contributed to this report.