President Donald Trump and Senator Richard Blumenthal, Democrat-Connecticut, have at least one thing in common.

Both men received multiple draft deferments during the Vietnam War, as able-bodied men were deployed by the thousands to fight a wildfire conflict during the Cold War. Blumenthal received five deferments from 1965-1970. Trump received four student deferments by 1968.

Their deferment histories diverged at this point. Blumenthal enlisted in the Marine Corps Reserve in 1970, serving six years in the United States.

Trump received his medically disqualifying deferment two years earlier, in 1968.

Those wartime histories reemerged on Monday, as Trump sent out a multi-pronged tweet assault during his 17-day working vacation, attacking Blumenthal's misleading 2010 comments when he said he served in Vietnam.

The tweets are yet another moment in which Trump, who is generally supported by veterans and staffed his cabinet and advisers with senior officers, has opened himself to criticism over the military, including from conservatives.

And his own record of missing the draft is fraught with unanswered questions over the legitimacy of his condition.

Trump's tweets came after Blumenthal voiced support on CNN for continuing the investigation into Russian meddling in last year's election and expressed concern about the Justice Department's increased focus on rooting out administration officials who leak information damaging to the president.

Trump responded shortly after on Twitter.

"Never in US history has anyone lied or defrauded voters like Senator Richard Blumenthal," Trump wrote. "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."

Blumenthal came clean in a 2010 news conference following allegations he lied about his record in an effort to appeal to voters, saying he meant to say "during" Vietnam and not "in" the country.

"On a few occasions I have misspoken about my service, and I regret that and I take full responsibility," he said, flanked by veterans. "But I will not allow anyone to take a few misplaced words and impugn my record of service to our country."

In response to Trump's tweets, Blumenthal wrote on Twitter: "Your bullying hasn't worked before and it won't work now. No one is above the law."

Trump has previously attacked Blumenthal on the same issue. In May, Trump went after Blumenthal when the senator criticised the president during television appearances for his firing of then-FBI Director James Comey.

Trump has made attacking the service of political opponents a cornerstone of his brash style while stumbling into self-created controversies.

On the campaign trail in 2015, Trump drew intense scrutiny after he attacked Senator John McCain, Republican-Arizona, a Navy pilot who was shot down over Vietnam, tortured for years and refused early release, which earned him admiration in the US and further beatings by his guards.

"He's a war hero because he was captured. I like people that weren't captured," Trump said, in a moment that threatened to derail his campaign. When McCain announced he was recovering after his brain cancer diagnosis in July, Trump said: "Senator John McCain has always been a fighter."

In January 2016, then Republican presidential candidate Trump pledged to give $1 million of his own money during a nationally televised fundraiser for veterans' causes, but only forked over the money four months later to the Marine Corps-Law Enforcement Foundation after being pressed for details by The Washington Post and other media organisations.

Trump also maligned the Muslim family of a dead American soldier, called the first general to counter the Islamic State a failure, and on numerous occasions, after running on a platform claiming credibility with military and veterans issues, he incorrectly referred to the 'Veterans Administration' on the campaign trail and as President-elect. The Veterans Administration was dismantled before the Berlin Wall in 1989 and became the Department of Veterans Affairs.

When Trump registered for the draft at 18 in 1964, he had just graduated as a cadet from the New York Military Academy in Cornwall, where he played football and basketball.

On the campaign trail, Trump would claim it was for bone spurs in both feet that prevented his service in Vietnam. His campaign said it was due to his high draft lottery number in 1969, though his medical deferment remained in place until 1972, when he received a permanently disqualifying deferment - a year before the draft ended and the war began to wind down.

Downplaying a a political foe's military service is a typical Washington tactic. But Trump's criticisms of McCain and Blumenthal has come even after his raunchy conversations with shock jock Howard Stern in 2004 revealed a particularly detached perspective on military service from the billionaire real estate magnate.

"I've been so lucky in terms of that whole world. It is a dangerous world out there - it's scary, like Vietnam," Trump told Stern about the effort to avoid sexually transmitted diseases in the 80s and 90s, as the AIDs crisis blossomed.

"It is my personal Vietnam. I feel like a great and very brave solider," Trump said, to laughs and egging on by Stern.

"I'm old enough to have gone through the 70s," Trump said.

And he was. Trump was in Manhattan in the 1970s, spinning his inheritance into real estate gold. Blumenthal was stateside in a Marine Corps uniform until 1976. McCain was a guest against his will at the Hanoi Hilton for five years until his release on 14 March 1973.

McCain regained his ability to walk the spring before Trump waltzed out of a review board with a medically disqualifying deferment on 17 September 1968.

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Units in Vietnam could have used Blumenthal and Trump that year, which kicked off with the bloody Tet Offensive. It was the deadliest year of the war, with 16,899 US troops killed.

The names of those dead, contemporaries of both men, were later etched into black granite in Washington, memorialising those who answered the call.