The presidential campaign featured a battle over military brass Tuesday, as Republican Donald Trump released a letter from 88 retired generals and admirals while Hillary Clinton called Trump's campaign an 'insult' to servicemen and women.

Scores of retired generals and admirals have come out in support for Trump, saying the billionaire Republican candidate should be the next commander-in-chief.

'These are our fighting generals. There are actually a lot more to come,' Trump told a crowd in Virginia Beach that was stacked with ex-military members.

Clinton counterattacked, speaking to reporters for the second day in a row with reporters traveling with her on her campaign plane.

Donald Trump has received the backing of 88 retired military figures

'His whole campaign has been one long insult to all those who have worn the uniform,' Clinton said at the University of South Florida in Tampa.

'I think we’re up to 89, who's counting,' Clinton said, diminishing Trump's letter. She said 2008 GOP nominee John McCain's and 2012 GOP nominee Mitt Romney's campaigns had between 300 and 500 retired officers in their corners.

‘But here’s what’s important: I now have more endorsements from retired flag officers than any Democrat, other than an incumbent president, has ever had. Compare where Trump is with where both Romney and McCain were,' she said.

‘I’m doing better than any Democrat. He’s doing worse than recent Republicans,' Clinton added.

In addition to flag officers, ‘we have many national security professionals. And a lot of the people who are endorsing me have never endorsed before, or they certainly have never endorsed a Democrat,' Clinton added.

In a letter published on Tuesday, 88 former military figures - including a Holocaust survivor - said they trusted Trump to 'rebuild our military, to secure our borders, to defeat our Islamic supremacist adversaries and restore law and order domestically.'

The letter did not mention Hillary Clinton by name but it took a sideswipe at the former secretary of state, saying that the army needs someone who has not been responsible for 'the hollowing out of our military'.

The letter is a boost for Trump and comes the same day as a CNN/ORC poll put him two points ahead of his bitter rival Clinton

'For the past eight years, America's armed forces have been subjected to a series of ill-considered and debilitating budget cuts, policy choices and combat operations that have left the superb men and women in uniform less capable of performing their vital missions in the future than we require them to be,' it went on.

The Obama administration has been criticized for overseeing a shrinking of the Pentagon's budget - a cumulative 15 percent since 2011, according to Politifact.

Trump cheered the news on his website.

'It is a great honor to have such amazing support from so many distinguished retired military leaders. I thank each of them for their service and their confidence in me to serve as commander-in-chief,' he wrote.

'Keeping our nation safe and leading our armed forces is the most important responsibility of the presidency.

'Under my administration, we will end the weak foreign policy of the last eight years, rebuild our military, give our troops clear rules of engagement and take care of our veterans when they come home.

'We can only Make America Great Again if we ensure our military remains the finest fighting force in the world, and that's exactly what I will do as president.'

The letter is a boost for Trump and comes the same day as a CNN/ORC poll put him two points ahead of his bitter rival.

The signatories also hit back at repeated claims from Democrats that the businessman is volatile and impulsive and so not to be trusted with nuclear codes.

One signatory, retired Major General Sidney Shachnow, is the only Holocaust survivor to become a U.S. army general.

'He has the temperament to be commander-in-chief,' Major General Sidney Shachnow stated, according to Trump's website. Shachnow is the only Holocaust survivor to become a U.S. army general.

The issue of defense has become a hotly fought topic between Clinton and Trump. The Democrat presidential hopeful on Tuesday released a new TV advertisement entitled Sacrifice.

The ad shows military veterans watching some of Trump's more provocative statements, including his claim to know more about Islamic State than military generals, and his criticism of Senator John McCain, a former prisoner of war.

It also features Trump's claim that he sacrificed a lot compared to families who have lost loved ones in conflict.

'Our veterans deserve better,' reads a line at the end of the ad, which is airing in Ohio, Florida, Iowa, Nevada and Pennsylvania.

Obama's spending bill last fall was set to grow the defense budget once more, by around 6 per cent.

Politifact attributes the Obama administration's reduced military budget partly to the removal of troops from Iraq and Afghanistan, and partly to sequestration: across-the-board cuts that automatically came into place when both parties failed to strike a spending deal in Congress in 2011.