Mr Clegg did radio interviews as he continued campaigning
The Lib Dems say it is "nonsense" to suggest they cannot be trusted on national security, after criticism from former defence and intelligence chiefs.
Peter Clarke, Sir Richard Dearlove and General Lord Guthrie said their Trident policy was a "colossal gamble".
Senior Lib Dem Ed Davey told the BBC the timing of the attacks, two days off an election, was "highly questionable".
He said the letter was "riddled with errors" and Lord Guthrie supported them on the nuclear weapons system.
Mr Clarke, the former national anti-terrorism co-ordinator, Sir Richard, the former chief of MI6 and Lord Guthrie, former chief of the defence staff, questioned Lib Dem policies on Trident, Nato, Afghanistan and counter-terrorism in a letter to the Times.
They wrote the party had "a wide range of positions" on withdrawing troops from Afghanistan, were "shy" of putting forward new anti-terror proposals and suffered from "potentially dangerous confusion" over its policy on the Trident nuclear missile system.
And Lord Gilbert, a former Labour defence minister, told BBC Radio 4's World at One programme he was so concerned about the "frivolous" attitude of the Lib Dems to the "defence of the realm" he advocated tactical voting against them.
"It will be up to individual Labour voters to decide, if they are in constituencies where they think their vote is going to be wasted because the Labour party is in third place, I think they ought to vote for the party which, in their view, is going to be most responsible in terms of defence of the realm.
"I don't think there's any doubt where their choice would fall. It would have to be the Conservatives if the choice is purely between the Conservatives and Liberal Democrats."
But Mr Davey said the idea the Lib Dems could not be trusted on defence was "pure nonsense".
'Riddled with errors'
He said the party did not want to scrap Britain's independent nuclear deterrent, they just did not want a "like for like replacement of a Cold War nuclear deterrent".
He said many of the generals agreed and the Lib Dems were more "in tune" with US President Barack Obama's nuclear strategy.
He said the letter to the Times was "questionable" in its timing and "riddled with errors" and he criticised its authors.
Lord Guthrie supported them on Trident, he said, Sir Richard Dearlove was "the man who was head of the intelligence services which told Tony Blair there were weapons of mass destruction in Iraq". He said only the Lib Dems had "stood up to that nonsense".
And he said Mr Clarke had put forward one of the "most authoritarian draconian" proposals ever - that terrorism suspects should be detained for up to 90 days before they had to be charged.
"I'm proud that the Liberal Democrats stood up for civil liberties on those issues against Peter Clarke and stood up against the Iraq War, against Sir Richard Dearlove."
Former Lib Dem leader Lord Ashdown also dismissed the letter as "a last-minute election stunt to frighten the electorate into voting for the Tories".
The Lib Dems are continuing to target traditional Labour constituencies in Liverpool and Glasgow on Tuesday, arguing that Labour have "betrayed" people and they are the only "progressive" alternative.
Mr Clegg's focus is on "fairer policies", including an extra £2.5bn investment in schools.