Blumenthal is an archetypal Clinton friend and confidant, someone Bill and Hillary first met 30 years ago and an informal aide who has been by the family's side during difficult moments, including Bill Clinton's impeachment trial.
The impeachment trial, according to close friends, solidified Blumenthal's relationship with the Clintons. Blumenthal routinely provided Bill and Hillary Clinton with information about their Republican opponents during the process and how to message against them.
"I think they both believe that the role he played in the White House then, especially in crafting political defenses, was important in the president surviving impeachment and staying in office," said someone with knowledge of their relationship who did not want to speak on the record about personal matters.
Blumenthal, a former journalist, worked in the Clinton White House as senior adviser from 1997 to 2001. He also works for Media Matters, a liberal news watchdog organization that looks to counter conservative claims in the media.
During his 30 years of associating with the Clintons, Blumenthal gained a reputation as a bulldog, willing to do just about anything to defend the political family. In the 1990s, some newspapers referred to Blumenthal as "Sid Vicious," a nod to his no-holds-barred reputation and, at times, conspiratorial views.
Republicans have focused on Blumenthal during Clinton's testimony on Thursday, including Chairman Trey Gowdy of South Carolina, who used nearly all of his questions to drill on the Clinton confidant. Their main question: Why did he have personal access to Clinton while security experts did not?
Blumenthal sent Clinton dozens of emails on various foreign policy topics, including Libya, some of which were unsolicited and others that were requested by Clinton.
"Madame Secretary, Mr. Blumenthal wrote you 150 emails. It appears from the materials that we've read all of those reached your desk," Rep. Mike Pompeo said. "Can you tell us why security requests from your professionals ... None of those made it to you. But a man who was a friend of yours who'd never been to Libya, didn't know much about it ... every one of those reports that he sent on to you that had to do with situations on the ground in Libya, those made it to your desk."
Pompeo, of Kansas, also faulted Clinton for giving Blumenthal more personal access to her than Ambassador Chris Stevens, asking the former secretary of state whether the slain diplomat had her personal email, phone number, fax numbers and home address, all means of contact that Blumenthal had.
Blumenthal told reporters in June
after testifying behind closed doors for the committee that he did not actually write the memos he sent Clinton and accused the committee of targeting him because of his relationship with the family.
"So why was I subpoenaed at all before this committee? I am a longtime friend of Hillary Clinton. It seems obvious that my appearance before this committee was for one reason and one reason only, and that reason is politics," he said.
One reason Blumenthal's emails to Clinton about Libya have raised eyebrows is because while he was emailing Clinton, he was advising business interests in Libya and also was doing work for the Clinton Foundation, the Clinton family's sizable philanthropic endeavor.
After he testified, Blumenthal told reporters his work for the foundation had "nothing whatsoever to do with my emails to my friend."
Clinton said Thursday that she "did not ask (Blumenthal) to send me the information that he sent me."
But that is not totally true: Some of Clinton's emails have been released and, at times, the former secretary of state asked her friend to keep sending her notes and intelligence.
"Another keeper, thanks and keep them coming," she wrote in response to one email about Libya in 2012. "Thx for helping keep me informed along the way," she wrote in response to a Blumenthal note about politics in the United Kingdom.
Clinton seemingly valued Blumenthal's opinion, too, because she wanted the friend to join her as an adviser at State. But the move was blocked by Obama aides, reportedly because of his opposition to the President during the 2008 campaign.
Clinton said on Thursday that she did not know who blocked Blumenthal from joining her at State.
This is not the first time Blumenthal has been dragged into a congressional inquiry on behalf of the Clintons.
During a 1999 deposition for Bill Clinton's impeachment trial, Blumenthal testified that he had no knowledge or involvement in spreading derogatory information about Monica Lewinsky, the White House intern who carried on an affair with the 42nd president. Blumenthal was called out on that claim by journalist Christopher Hitchens, who filed a sworn affidavit at the time that accused Blumenthal of spreading rumors.
Newer Clinton friends have openly questioned why the political family stays close to people like Blumenthal, someone whose history has become a political thorn in their side.
But the Clintons are fiercely loyal, especially to those men and women who stood by them during some of the more harrowing moments in their careers.
"I'm going to keep talking to my old friends, whoever they are," Clinton said during a May press conference in Iowa. "He's been a friend of mine for a long time. He sent me unsolicited emails, which I passed on in some instances, and that's just part of the give-and-take."
Clinton added: "When you're in the public eye, when you're in an official position, I think you do have to work to make sure you're not caught in a bubble and you only hear from a certain small group of people."