The two men have a storied and complicated relationship that started well before Trump took office.
Rival real estate empires
Blumenthal's career started in law and then merged into government and public service, but his familial ties put his relatives in Trump's real estate orbit.
Blumenthal married Cynthia Malkin in 1982, and in doing so, he became a part of a real estate empire.
Cynthia Malkin's father Peter runs a real estate company, assets from which help contribute to Blumenthal's own net worth. In 2015, Blumenthal, regularly listed among the wealthiest members of Congress, was ranked by Roll Call as the eighth richest lawmaker with a net worth of $66.99 million.
Trump and Peter Malkin have had business run-ins well before Blumenthal started publicly criticizing Trump in the political arena.
Trump and Malkin's most public battle over partial ownership of the Empire State Building was drawn out for years.
According to a 1998 article in The New York Observer, Trump sued Malkin and Leona Helmsley, who jointly controlled the iconic building's lease. Trump argued that Malkin and Helmsley "let the skyscraper deteriorate into a 'roach-infested' commercial slum swarming with second-rate tenants," according to the article. The suit was later dismissed.
Military service jabs
In 2010, when then-Connecticut Attorney General Blumenthal was running to fill the state's vacant Senate seat, The New York Times reported that there had been several instances where Blumenthal either said or suggested that he served during the Vietnam War.
What actually happened was that Blumenthal received at least five military deferments that stopped him from being sent to the war. In 1970 he joined the Marine Reserves. During that time, he never went to Vietnam.
In spite of that history, there were several instances when Blumenthal misrepresented the truth. In a 2008 speech, he referred to "the days that I served in Vietnam," and in news stories from around that time, he didn't correct articles that referred to him as "a Vietnam veteran," according to The Times.
After The Times story was published, Blumenthal apologized, saying, "I have made mistakes and I am sorry."
"At times when I have sought to honor veterans, I have not been as clear or precise as I should have been about my service in the Marine Corps Reserves," he said in a statement issued May 23, 2010. "I have firmly and clearly expressed regret and taken responsibility for my words."
Trump has never served in the military, but said during the presidential campaign that "I've regretted not serving in many ways."
Like Blumenthal, Trump received four student deferments and then one medical deferment after being diagnosed with bone spurs in his heels, according to a New York Times report from August 2016.
His own deferments have not stopped Trump from attacking Blumenthal's misstatements, at one point in May going so far as to calling for him to be "investigated for his acts."
Blumenthal and Trump's disagreements turned political during the confirmation process of Supreme Court Justice Neil Gorsuch.
Blumenthal met with Gorsuch before his confirmation hearings, and then spoke about their conversation publicly.
"There is no question that Judge Gorsuch said that these attacks on the judiciary are disheartening and demoralizing," Blumenthal told ABC News in February about the conversation.
Blumenthal stressed that White House staff members were in the room at the time and that the judge was "very clear it was fine to share it publicly." In fact, he said Gorsuch seemed to encourage it.
The White House and Trump then claimed that Blumenthal had misrepresented Gorsuch's comments, even though Gorsuch's spokesperson had confirmed Blumenthal's characterization.
On Feb. 9, Trump tweeted: "Sen. Richard Blumenthal, who never fought in Vietnam when he said for years he had (major lie), now misrepresents what Judge Gorsuch told him?"
Sen.Richard Blumenthal, who never fought in Vietnam when he said for years he had (major lie),now misrepresents what Judge Gorsuch told him?— Donald J. Trump (@realDonaldTrump) February 9, 2017
Blumenthal is co-sponsoring legislation that will make it harder for the president to fire special counsel Robert Mueller who has been tasked with investigating any potential collusion during the presidential election.
Trump posted critical tweets of Blumenthal after the senator spoke about the proposal on a morning show.
"Interesting to watch Senator Richard Blumenthal of Connecticut talking about hoax Russian collusion when he was a phony Vietnam con artist!" Trump wrote in a tweet Monday morning.
Interesting to watch Senator Richard Blumenthal of Connecticut talking about hoax Russian collusion when he was a phony Vietnam con artist!— Donald J. Trump (@realDonaldTrump) August 7, 2017
"Never in U.S. history has anyone lied or defrauded voters like Senator Richard Blumenthal. He told stories about his Vietnam battles and conquests, how brave he was, and it was all a lie. He cried like a baby and begged for forgiveness like a child. Now he judges collusion?" he wrote in two other tweets.
Never in U.S.history has anyone lied or defrauded voters like Senator Richard Blumenthal. He told stories about his Vietnam battles and....— Donald J. Trump (@realDonaldTrump) August 7, 2017
...conquests, how brave he was, and it was all a lie. He cried like a baby and begged for forgiveness like a child. Now he judges collusion?— Donald J. Trump (@realDonaldTrump) August 7, 2017
"I think Senator Blumenthal should take a nice long vacation in Vietnam, where he lied about his service, so he can at least say he was there," Trump later tweeted.
I think Senator Blumenthal should take a nice long vacation in Vietnam, where he lied about his service, so he can at least say he was there— Donald J. Trump (@realDonaldTrump) August 7, 2017
Blumenthal responded Monday on Twitter, writing, "Mr. President: Your bullying hasn't worked before and it won't work now. No one is above the law."
"This issue isn't about me - it's about the Special Counsel's independence and integrity," he wrote in a second tweet.
This issue isn't about me - it's about the Special Counsel's independence and integrity.— Richard Blumenthal (@SenBlumenthal) August 7, 2017
Blumenthal also defended his tweets this morning.
"I cannot explain the president's obsession with me or any of the other targets of his tweets," Blumenthal said Tuesday on MSNBC's "Morning Joe."
He went on, "It's really not about me. And I have no idea what's in the president's head. But I can tell you I'm determined not to be distracted. I think they are designed to distract: these bullying tweets only reinforce my determination to protect the special counsel, because he has used the same tactics in seeking to bully and intimidate the special counsel Bob Mueller."