WASHINGTON — When Donald Trump was constructing the opulent Trump Tower in Midtown Manhattan four decades ago, he was infuriated when he saw a thin layer of golden-hued marble lining the walls and column in the lobby, and ordered aides to make it appear twice as thick.

Sure, he had architects and engineers to handle those decorating details so that he could focus on the building’s multimillion-dollar budget and other big-picture concerns of a business empire that would teeter in and out of bankruptcy.

But when something bothers Trump, however small, he can obsess over it.

That tendency to become preoccupied by narrow interests is haunting him in the impeachment inquiry, which hit a milestone Wednesday when the Democratic-led House Intelligence Committee held its first public hearing since the investigation began in September.

A second hearing is scheduled Friday, and eight more witnesses will testify over three days next week.

The first hearing provided compelling evidence of one of Trump’s most audacious fixations: getting Ukraine’s new president to announce investigations of Trump rivals, including potential 2020 opponent Joe Biden, after Trump had suspended $391 million in congressionally approved security aid to the government in Kyiv.

House Democrats argue that the evidence shows Trump hijacked foreign policy, and put national security at risk, to help his reelection bid. On Thursday, House Speaker Nancy Pelosi went further, saying for the first time that the president’s demands to Ukraine amounted to bribery.

“The bribe is to grant or withhold military assistance in return for a public statement of a fake investigation into the elections. That’s bribery,” Pelosi told a news conference.

During the hearing Wednesday, William B. Taylor Jr., the top U.S. diplomat in Ukraine, revealed publicly for the first time that an embassy staffer had overheard Trump speaking to the U.S. ambassador to the European Union, Gordon Sondland, who had called the president on a cellphone from a restaurant in Kyiv after meeting senior Ukrainian officials.

Taylor said the aide, who was later identified as political counselor David Holmes, specifically heard Trump ask Sondland about “the investigations,” and that Sondland said after the call that Trump cared more about Biden than about U.S. policy toward Ukraine, an ally battling a Russian-backed insurgency.

On Thursday, the Associated Press reported that a second U.S. diplomat, a foreign service officer based at the embassy in Kyiv, also heard Trump speaking on the call.

If confirmed, the cellphone conversation could place Trump and Sondland in jeopardy.

It contradicts Sondland’s previous sworn testimony about his direct interactions with the president, when he failed to mention the conversation, and potentially puts him at risk of perjury. Sondland is scheduled to testify publicly next Wednesday.

More importantly, it places the president more directly into the alleged scheme to demand political favors from Ukraine in exchange for U.S. assistance, a narrative that largely has been outlined by White House aides and U.S. diplomats so far.

As a side issue, it raises questions about why Trump was willing to risk security by taking a cellphone call from abroad.

Sondland’s call normally would be routed through a senior member of the national security staff or an assistant secretary of State, who might brief the president through a memo or verbally to a superior, who would then pass the information to the president.

“Nothing is particularly orthodox” in the way Trump runs diplomacy, said Alexander Vershbow, a former U.S. ambassador to Russia.

Sondland’s ability to dial up the president in a public place was particularly alarming to U.S. security experts, who cited Russia’s well-documented efforts to undermine the fragile democracy in Ukraine, and its near-certain surveillance of American diplomats there.

“The fact that (Sondland) either didn’t know or knew but took a cavalier attitude — neither answer is a really good answer,” said Larry Pfeiffer, a former senior U.S. intelligence official who ran the White House Situation Room from 2011 to 2013.

Pfeiffer said calls to the president from a foreign country are supposed to be made from a secure area in an embassy to the White House switchboard or the Situation Room, and then patched through to the president.

He said President Barack Obama received some personal calls on his BlackBerry device from a very small group of friends, but they were not supposed to contain potentially sensitive information.

Placing a call from a restaurant is particularly brazen. Even with the best equipment, government employees are constantly warned that “your call is only as secure as the place you’re sitting,” Pfeiffer said.

Republicans contend that Trump did nothing wrong, or at least worthy of impeachment, and suggested that his aides may have been acting on their own. They also complain that few of the witnesses had direct conversations with Trump, and derided their accounts as hearsay.

For his part, Trump retweeted a Fox Business host who said the hearing amounted to a “policy dispute” that average Americans would not find impeachable.

In some ways, the cellphone conversation, as described, would confirm available evidence about Trump’s preoccupation with getting Ukraine to investigate his political opponents.

On July 25, a day before Sondland picked up his cellphone, Trump had asked Ukrainian President Volodymyr Zelenskiy in a phone call for a “favor” immediately after Zelenskiy had pleaded for U.S. anti-tank weapons.

Trump made clear that he wanted Zelenskiy to investigate a debunked theory that Ukraine had interfered in the 2016 presidential election, and to reopen an inquiry into Burisma, the Ukrainian natural gas company that had put Biden’s son Hunter on its board.

Since Biden effectively led U.S. policy to Ukraine as vice president, the arrangement appeared to pose a conflict of interest. But no evidence has emerged to suggest that Biden or his son committed a crime, and Trump has never said why, if he believes such evidence exists, that he asked a foreign power instead of the Justice Department to investigate U.S. citizens.

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