WASHINGTON — Like most Republican senators sitting as jurors in President Donald Trump’s impeachment trial, Rob Portman of Ohio has sat quietly in the Senate chamber and studiously avoided speaking to reporters outside.

Unlike his colleagues, Portman has been credited by the president himself with convincing him to release a hold on the nearly $400 million in aid to Ukraine that is at the center of the House’s impeachment case and was involved in promoting a disputed narrative around the president’s motives.

In the initial days after Congress began investigating Trump, Portman helped validate a justification for the delay that has not been born out by the evidence, even as the Trump legal team has mounted an 11th-hour effort to resurrect it.

Trump’s lawyers have laid out a strategy to defend the president’s actions in part by suggesting he blocked the aid because European nations have failed to provide adequate financial assistance toward Ukraine’s security. That argument has largely been debunked — numerous fact checkers have noted that Europe has provided two-thirds of the aid since Russia illegally annexed Crimea in 2014.

But it’s an argument Portman echoed in television appearances last fall and it’s one that contradicts the main rationale Trump has offered since then: that the freeze was due to “corruption” concerns in Ukraine.

Portman was among the last known people outside of the White House to speak with Trump before the aid was released in September, according to a White House brief, and he’s spoken little publicly about his role since last fall. A day after it was unfrozen, Portman thanked the president and said he agreed with Trump’s assessment that other nations need to step up their assistance.

And the president singled out the Ohio senator as one of primary movers behind getting the aid released. "I gave the money because Rob Portman and others called me and asked," Trump told reporters in early October.

As part of the Senate trial, Portman has already voted against releasing correspondence from the Office of Management and Budget that could contradict the argument around Europe’s contributions. And his participation in the disputed narrative is increasingly relevant as he faces a vote on whether to call witnesses, including former national security adviser John Bolton, who could establish whether Trump was telling Portman the truth about his motives.

Portman was among just seven senators expressing opposition to impeachment before any witnesses had testified in the House. That’s in contrast to other Republicans representing swing states who had yet to weigh in.

In the immediate aftermath of the original whistleblower report that kickstarted the House investigation, Portman said the issue of corruption never came up in the conversation that allegedly led to the release of the aid.

“The president was very clear with me. He only raised one issue, and that issue only, and that was about Europeans not doing enough. I don’t disagree with him on that,” Portman said in a Sept. 26 Fox News interview.

Importantly, Portman added, “He (Trump) never linked the aid to corruption in general or certainly not an investigation in particular.”

Portman in the past has been critical of Trump's July 25 phone call with Ukrainian President Volodymyr Zelenskiy in which Trump raised the issue of investigating former Vice President Joe Biden. “The president should not have raised the Biden issue on that call, period. It’s not appropriate for a president to engage a foreign government in an investigation of a political opponent,” Portman told the Columbus Dispatch in October, before the House impeachment hearings. Though Portman has also said he doesn’t believe there was a “quid pro quo.”

Portman was part of a small group who spoke with Trump the evening of Sept. 11, just before the aid was released, according to a White House impeachment brief. An experienced foreign policy hand, former White House budget director and chairman of the bipartisan Ukraine Caucus in the Senate, Portman would go on to press the argument about a lack of European contributions as Trump offered that as justification after news of the whistleblower report became public.

At the White House in October, Trump repeated the claim about Europe’s contribution, invoking Portman. "We were the sucker country for years and years,” Trump said. “But I gave the money because Rob Portman and others called me and asked. But I don't like to be the sucker and European countries are helped far more than we are, and those countries should pay more to help Ukraine.”

Yet David Holmes, a U.S. embassy official in Ukraine, testified in November that Trump’s assertion was wrong and communicated that to the White House at the end of August. Since 2014, the U.S. has provided a combined $3 billion while the Europeans have provided $12 billion. “That information showed a different story,” he testified.

Portman, through his spokesperson, declined an interview request from NBC News. But a senior Portman aide , who spoke on condition of anonymity because he was not authorized to speak publicly, acknowledged that his specific claim that Europe has fallen short in its financial obligations to Ukraine is incorrect and that his staff later reminded the senator of this.

But the aide added that Portman, a member of the Foreign Relations Committee and the Senate’s resident Ukraine expert, was referring to $250 million in military aid, specifically, and does believe that NATO should do more when it comes to Ukraine’s security. Portman was doing what he could to get the funds released, the aide said.

“All he did was relay the facts of what took place on the (Sept. 11) call,” the senior aide said.

Those comments put Portman at the center of a narrative that diverts from what the president and his allies are saying today.

While Trump did mention concerns about Europe contributing more to Ukraine in his July call with Zelenskiy, the president and his allies had largely dropped the argument about Europe to focus instead on concerns over corruption, until his defense team resurrected it this week.

Portman, who’s been quiet about his role as Ukraine Caucus chair amid impeachment proceedings, notably tweeted after meeting with Zelenskiy in May that the two “discussed his (Zelenskiy’s) positive agenda to fight corruption.” Portman then repeatedly praised Ukraine’s new leader as a reformer, including in a June speech on the Senate floor. On Fox, he said Zelenskiy was “doing all the right things” and “needs the help.”

Portman, in many ways, should be an ideal character and fact witness for Trump. He’s a former budget chief under President George W. Bush who is widely viewed as a thoughtful pragmatist and institutionalist. As a leading advocate for Ukraine, he also has a unique understanding of the stakes in ensuring that country’s security and the levers in the budget office that need to be pulled.

In the weeks that followed the September 11 call, Trump publicly spoke of his concerns about Europe’s contribution, while Portman echoed those claims in media interviews.

On Sept. 24, while attending the United Nations General Assembly, Trump told reporters about his concerns about Europe to justify withholding aid. The same day, in an interview on Fox Business, Portman added his voice in support.

“The president is correct in that. And that was the reason that I was given as to why the funds were not being released,” Portman said. “And I agree with that, that we needed to be doing more to pressure our European friends, and NATO in particular, to help Ukraine.”

Portman added: “But my point was, ‘Gee, don’t take it out on Zelenskiy,” who “needs our help.”

For most of the House impeachment hearings, the president and his defenders largely dropped the Europe narrative, instead focusing on rooting out “corruption” as they sought to create a premise for investigating the Bidens, although they have touched on it at times. “The president is focused on burden sharing, and corruption,” Trump defense attorney Mike Purpura said in his Jan. 25 opening argument.

White House budget official Mark Sandy testified in the House that he’d heard nothing about European contributions to Ukraine as playing a role in the hold up until right after the whistleblower report made its way to Congress. Laura Cooper, the Defense Department’s Ukraine expert, also testified she has “no recollection” of European “burden sharing” coming up in three separate interagency meetings she attended.

“The only logical conclusion, based on all this evidence, is that the president lifted the hold on Sept. 11 because he got caught,” Rep. Jason Crow, D-Colo., among the House impeachment managers argued on Friday. “The White House attempted to create a cover story for the president’s withholding of the assistance,” he said.