What doomed Richard Nixon was the release of an audio recording from the Oval Office in which he could be heard instructing aides to carry out the coverup of the White House’s involvement in the Watergate break-in. Within days of the tape being made public, Nixon boarded Marine One on his way back to private life.

Nixon was already in trouble, mind you. Congress was pushing forward with impeachment, and his support on Capitol Hill and in the public had eroded badly. But the tape removed any doubt that Nixon himself was responsible for what happened. The line was direct and unquestionable, and Nixon was gone.

For all of the evidence to cascade out of the impeachment inquiry focused on President Trump since it began at the end of September, no similar smoking gun has been wrung from witnesses. Trump’s involvement in the apparent effort to cajole Ukraine into launching investigations that would politically benefit the president has been almost exclusively conveyed through secondhand testimony so far. A lot of that testimony is fairly straightforward, of course, and Trump’s own embrace of his actions as broadly A-okay has been pretty far from a denial of wrongdoing. But those looking to defend Trump have a fairly straightforward way to do so: Anything done wrong was done without Trump’s knowledge.

That appears to be exactly where his defenders on Capitol Hill are headed. Evidence that the administration was using a White House meeting or aid to Ukraine as leverage to spur the investigations has directly implicated Gordon Sondland, the U.S. ambassador to the European Union, and acting White House chief of staff Mick Mulvaney — since both of whom have admitted to doing so. But there’s no scratchy recording of Trump ordering Mulvaney to twist Ukraine’s arm and no email to Sondland demanding he do the same. Trump’s long-standing habit of avoiding written documentation has been an asset that the record-everything Nixon would have been wise to emulate.

This presentation of the situation underplays the extent of evidence linking Trump directly to the effort to influence Ukraine. In the abstract, one can see how an assertion that Trump hasn't been looped into the effort directly might be rationalized. But a slightly closer look at what is already known shows how difficult it will be to make that case.

It starts with the most obvious evidence of Trump’s involvement in the effort: the rough transcript of his July 25 call with Ukrainian President Volodymyr Zelensky. The transcript was released by the White House at the outset of the investigation, allowing Trump to subsequently claim that it was necessarily exonerating. In recent days, Trump has embraced “read the transcript” as a rallying cry. In part, his doing so is an effort to define the call as the only salient event to his administration’s interactions with Ukraine. But it’s more broadly intended to suggest that his innocence is established by the rough transcript itself.

It isn’t. Instead, the document shows Trump making the exact requests that have been identified in other witness testimony: his request that Ukraine investigate former vice president Joe Biden, by name, and that Ukraine probe any possible Ukrainian involvement in 2016 election interference. (That summary of the requests is generous in the implication that there’s something to uncover; there’s little evidence that there is.) The 2016 request, in fact, followed Zelensky’s mentioning military aid which Trump knew was already on hold at his direction; Trump replied that the U.S. would like “a favor, though."

It was only after Zelensky agreed to launch investigations that Trump extended an invitation to the White House. It brings to mind a comment made by acting Ukraine ambassador William B. Taylor in his testimony to House investigators: At one point, Sondland reminded Taylor that Trump was a businessman, and that businessmen only signed checks once they had the product in hand. On that call with Zelensky, the Ukrainian president handed over the investigations, and Trump only then paid him with the invitation.

That particular exchange is important because it is precisely the exchange suggested in a text-message exchange before the call between then-special U.S. envoy to Ukraine Kurt Volker and Andrey Yermak, an adviser to Zelensky.

“Heard from White House — assuming President [Zelensky] convinces trump he will investigate / ‘get to the bottom of what happened’ in 2016, we will nail down date for visit to Washington,” Volker wrote. Asked by inquiry investigators who at the White House he’d heard from, he identified Sondland — who had texted him the morning of the call with an urgent request that Volker get in touch. In an interview on July 26, Sondland said that he’d talked to Trump shortly before the call, making a Trump-to-Sondland-to-Volker-to-Yermak-to-Zelensky backchannel something less than unlikely.

Sondland himself has been careful about implicating Trump. When his testimony before the impeachment inquiry was released earlier this week, he admitted to having told Yermak on Sept. 1 that military and economic aid to Ukraine, which had been halted a week before the Trump-Zelensky call, wouldn’t be delivered unless new probes were announced. But he claimed that this wasn’t a link he made at the direction of Trump; instead he claims to have just assumed that was the requisite condition.

As we’ve reported, there was no other information provided about the rationale for stopping the aid, including to Ukraine which would presumably have been given benchmarks to meet if the aid was halted out of concern that the country wasn’t taking certain actions. The announcement that aid was being stopped came in a conference call on July 18, with a staffer for the White House Office of Management and Budget saying only that the stoppage came at the direction of Trump via Mulvaney.

Mulvaney was also the reported trigger for Sondland’s insistence during a meeting with Ukrainian officials (including Yermak) in early July that a meeting would only occur if Ukraine launched the desired probes. At a news conference last month, Mulvaney admitted that an investigation into purported 2016 interference was a predicate for resuming aid. Mulvaney is, again, Trump’s acting chief of staff.

Sondland, Volker and Energy Secretary Rick Perry were tasked by Trump during a May meeting with operating an informal Ukraine diplomatic effort, coordinating with Trump’s personal attorney Rudolph W. Giuliani. They relished the role, dubbing themselves the “three amigos,” and coordinated on several recorded occasions with Giuliani directly. Volker connected Giuliani to Yermak, for example, and Sondland and Volker looped Giuliani into their efforts to draft a statement that Zelensky would release announcing new probes.

Sondland has claimed that he didn’t know that one of those investigations, centered on an energy company called Burisma Holdings, had anything to do with Biden. He told investigators that he would “not endorse investigating the Bidens.”

By May 23, when Trump tasked the “amigos” with running point on Ukraine — in effect establishing a parallel diplomatic track, to acting ambassador Taylor’s consternation — Giuliani’s focus on the Bidens was already well-established. He made national headlines earlier that month when he announced that he planned to go to Ukraine to investigate his Biden theories directly, only to cancel the trip in the face of strong pushback given the appearance that Giuliani was seeking to dig up politically useful dirt for Trump.

Giuliani told the New York Times that he hoped to meet Zelensky, then the president-elect.

“I am going to tell him what I know about the people that are surrounding him, and how important it is to do a full, complete and fair investigation,” Giuliani said, referring to conversations he’d been having for months, including with a former Ukrainian official who Biden and other officials had insisted be fired.

“I’m asking them to do an investigation that they’re doing already and that other people are telling them to stop,” Giuliani told the Times. “And I’m going to give them reasons why they shouldn’t stop it because that information will be very, very helpful to my client, and may turn out to be helpful to my government."

He’d already tried to help his government by giving the State Department a packet of documents including transcripts of interviews he and his team had conducted with Ukrainian officials. (Those interviews were conducted with his aides Lev Parnas and Igor Fruman, who were indicted last month for campaign-finance violations.) The packet was identified on the enclosing envelope as having come from “THE WHITE HOUSE."

Giuliani told the Times in May that Trump “basically knows what I’m doing, sure, as his lawyer."

Trump certainly knew what Giuliani was doing, in part because it was all over Fox News. Biden was mentioned in conjunction with Ukraine 50 times on Fox News in May alone. Trump himself gave an interview to a Fox News host on May 19 in which he specifically raised questions about Biden’s effort to get the prosecutor fired. Four days later, Trump met with Sondland, Volker and Perry and told them to work with Giuliani. Presumably if Trump objected to Giuliani’s overt, public efforts to pressure Ukraine, he might have pointed the “amigos” to seek direction somewhere else.

Giuliani's integration into the process was so thorough that he came up in a text message Yermak sent Volker right after the July 25 call.

Since Zelensky assured Trump that he’d launch the desired investigations in accordance with Volker’s pre-call direction, Yermak sought to set up dates for a Washington visit. He also asked Volker to "please remind Mr. Mayor to share the Madrid’s dates” — suggesting that he understood one next step was that August meeting with Giuliani.

Testimony provided to the House impeachment inquiry has included a number of times where efforts to get Ukraine to announce new investigations have been linked back to Trump.

Sondland told Volker in August that the team was on the brink of setting up a Zelensky visit once a statement about investigations was finalized — but not until then because Trump “really wants the deliverable” of an announcement. He also told Sen. Ron Johnson (R-Wis.) in late August that Trump was predicating aid on investigations. When Johnson raised this with Trump, the president said there was “no quid pro quo” — but also that it was important to get to the bottom of what happened in 2016. This is exactly the line Mulvaney tried to draw during his October news conference. It’s also the line Trump reportedly tried to draw in a conversation with Sondland in early September, which the National Security Council’s Tim Morrison told Taylor he reported to NSC attorneys.

After Sondland told Yermak on Sept. 1 that aid was contingent on investigations, Taylor asked him about it.

“Ambassador Sondland told me that President Trump had told him that he wants President Zelenskyy to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 U.S. election,” Taylor testified. “'Everything' was dependent on such an announcement,” he added, “including security assistance.”

This timeline makes clear why Trump’s allies want to pin everything on Mulvaney, Sondland and Giuliani. Each of them was a point of contact for efforts to pressure Ukraine into launching investigations, and there is only secondhand evidence linking those efforts back to Trump. Maybe, the argument goes, they were working collectively to deliver what they believed Trump wanted in a way to which he would have objected.

There are two problems with that theory, though. The first is that it requires assuming that a wide variety of testimony suggesting that Trump was involved in the effort contemporaneously is incorrect or untrue. It takes only one bit of additional documentary evidence showing Trump's direct involvement to undercut that idea.

The other problem, of course, is that the existing documentary evidence — that rough transcript — falls explicitly within the bounds of what’s alleged: Trump leveraging his position and American assets to get Ukraine to launch politically useful investigations. Reading the transcript, far from being exculpatory, demonstrates exactly how Trump’s actions over the summer fall into the pattern of pressure from which Trump’s allies hope to separate him.

By itself, it may be the smoking gun.