President Trump's former national security adviser, John Bolton, is known as a conservative foreign policy hawk. But he is turning into a key figure in the Democratic-led impeachment inquiry of Trump.

"All administrations are tempted to do bad things and have people who have bad instincts or wrong instincts," said Danielle Pletka, who oversees the conservative American Enterprise Institute's work on foreign policy and defense.

"And then there are people who just have judgment," she said, noting that every administration has "white hats and black hats," people who try to do the right or wrong thing. "And I would say that John Bolton comes out of this with a white hat of judgment."

Investigators in the impeachment inquiry on Capitol Hill have heard from U.S. diplomats and former White House officials about the president's effort to have a political rival investigated in Ukraine while the administration withheld military aid. Some witnesses have described Bolton as someone who pushed back.

But Bolton also hardly fits the profile of someone who would be in "The Resistance" to Trump. His saber-rattling, hard-line stances toward countries such as Iran have made him something of a boogeyman for liberals through the years, especially after the Sept. 11 terrorist attacks.

But now, after testimony about Bolton's role in the battle over the Ukraine affair, a more complicated portrait of the former national security adviser emerges.

"I think the portrait that comes through is exactly the person who he is," Pletka said. "He's a very shrewd lawyer. He's very attuned to his own principles, and he doesn't get mixed up in dirty stuff."

Lawmakers involved in the impeachment inquiry heard from two people who worked with Bolton on Ukraine. They said Bolton was so irritated after he learned about the plan to connect U.S. military assistance to a Ukrainian pledge to investigate conspiracies about the 2016 election and the Bidens that he abruptly ended a meeting.

He told Fiona Hill, a former National Security Council official in charge of Russia policy, that she should have "nothing to do with domestic politics." According to Hill, Bolton also called Rudy Giuliani, the president's personal lawyer and shadow emissary to Ukraine, a "hand grenade who's going to blow everybody up."

The man who took over for Hill on the National Security Council, Tim Morrison, is scheduled to testify Thursday. House committees conducting the impeachment inquiry have spoken with a lawyer for Bolton about a possible deposition, according to media reports citing unnamed sources. NPR has not confirmed the discussions.

William Taylor, the current acting ambassador to Ukraine, named Morrison, who is still with the White House, more than a dozen times in his deposition. He noted that he and Morrison each had knowledge of — and opposed — the withholding of aid.

Taylor also testified that Bolton was also against setting up a phone call between Ukrainian President Volodymyr Zelenskiy and Trump because he was worried it would "be a disaster."

It's now known, according to a record of the call released by the White House, that Trump asked Zelenskiy to "do us a favor though" — after talking about military aid — and investigate the two conspiracy theories, including the one about Hunter Biden, the son of Joe Biden, the former vice president and a 2020 presidential hopeful.

At the time of that phone call, military aid was being withheld from Ukraine, despite Congress already having approved the allocation of the money to the country. After all, it was U.S. policy to help Ukraine defend itself in an ongoing hot war against Russia, a war that had already claimed 13,000 Ukrainian lives.

The White House has continued to deny that there was a quid pro quo (though the acting White House chief of staff essentially said there was but then walked back his remarks). And some House Republicans say Taylor's testimony doesn't tell the whole story.

Taylor testified that there was a direct link between the aid and the desire for a promise of "investigations" — and that Trump had ordered it all.

Bolton found all of this troubling.

"John gets a lot of bad press, and I think, 'Thank God John was there,' " said Eric Edelman, a former undersecretary of defense for policy in George W. Bush's administration. "There are a lot of people who were very negative about John, but I think on all the big issues — North Korea, Iran, this Ukraine-Russia stuff — I think, by and large, John was on the side of the angels and trying to keep the president from going outside of bounds."

Going outside of bounds on foreign policy, but also potentially outside the bounds of the Constitution.

The idea of Bolton as a guardrail is not so outlandish. Back in June 2018, after Bolton was named national security adviser, Mark Groombridge, a former aide to Bolton, told NPR that he was one of many foreign-policy conservatives who was hopeful Bolton would be a kind of backstop against the president's most self-destructive instincts.

"To the extent that he can help shape or influence or minimize the damage that Trump potentially could do, the ambassador feels he has a patriotic duty to do so," Groombridge said of Bolton, who served as the U.S. ambassador to the United Nations under Bush. "And while John will salute and follow the president, he will have a very difficult time squaring his own personal beliefs and convictions with a deal that he thinks is fundamentally not in U.S. interests."

In this case, according to testimony before the House, the deal was the withholding of military aid until Ukraine's president publicly committed to investigating the Bidens.

There have always been two aspects to Bolton's reputation — he was a conservative hawk, not afraid to advocate for U.S. military action, but he was also a skilled infighter, a bureaucratic black belt — and that comes through in the testimony too.

He told Hill to "brief the NSC lawyers" to make sure there was a record of her opposition to the shadow foreign policy.

He told Taylor to send a first-person cable to Secretary of State Mike Pompeo that expressed Taylor's concerns.

Groombridge said all this is right in character.

"His bureaucratic skills served him very well in that there is now a paper trail, which essentially exonerates Ambassador Bolton and, in some ways, almost paints him as the hero in this. I mean, I recall the first day I joined the State Department — it was Oct. 15, 2001. And the only sort of fatherly advice Ambassador Bolton gave me at the time was, he said, 'Always get the process right. That way those who oppose you are forced to engage you on substance.' And I think Ambassador Bolton, in his capacity as [national security adviser], was doing the same thing."

The Ukraine affair isn't the first time Bolton disagreed with Trump during his tenure as Trump's national security adviser. Bolton wanted tougher lines on Iran, Venezuela and North Korea. Trump did not.

Back in September, when Trump fired Bolton by tweet, Bolton's enemies — and there were plenty of them — described him anonymously as abrasive, self-promoting, a war monger, disloyal.

And though Bolton, who was once a frequent presence on Fox News, has gone to ground, turning down repeated requests to discuss his role in the Ukraine scandal, he will likely respond. He told The Washington Post that he would have his say in "due course," and he is currently shopping a book deal.

Democrats on Capitol Hill have lots of questions for him, including, "Did Bolton ever bring his concerns to the president directly?" and "What other steps did he take to free up the aid to Ukraine?"

Congress has asked two of Bolton's former aides to testify next week. It's unclear if they'll appear, but the House impeachment inquiry will also, eventually, want to hear from Bolton himself.

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