(CNN) Donald Trump said Thursday that he was being sarcastic when he appeared to suggest that Russia should use espionage to find Hillary Clinton's deleted emails.

But he's finding out that presidents -- and by extension, presidential nominees -- don't get that luxury when the topic is national security.

The GOP nominee tried to dampen controversy over his apparent call Wednesday for Russia to either stage an espionage cyber hack to find Clinton's deleted emails or to publish information it had already stolen.

"Of course I'm being sarcastic," Trump said in a Fox News interview that aired the day after his comments at a news conference in Florida sparked a national furor and offered ammunition for Democrats who claim he's not fit to be president.

The affair is a lesson for Trump in how every word a potential commander in chief utters is parsed and amplified, and can have significant political and diplomatic consequences. US presidents in the modern era have seen singular sentences and offhand comments define global perceptions on US policies and leadership.

It's nothing new for the outspoken Republican nominee to cause a firestorm with comments that he made in a press conference; he's been doing it for his entire presidential campaign, with any resulting political damage seeming to be offset by the media attention and appeal they have to his voters.

But when they step up to accept their party's nomination, candidates move into an arena where the stakes are higher and the bar for mistakes is much more unforgiving than the rough-and-tumble of a primary campaign.

Nominees are viewed by voters, reporters, their peers and future international counterparts as commanders in chief-in-waiting on whose choice of words lives and crucial national security interests could ultimately depend. As a result, the room for error is far narrower than before.

That they are preparing to assume that role is underscored by the classified intelligence briefings they receive as official major-party nominees. A US intelligence official told CNN on Thursday that the process will begin within days.

On Thursday, Senate Minority Leader Harry Reid called for Trump to get "fake briefings."

"I think that the man is a loose cannon," he told CNN. "He's done so much to hurt our country with our international relations."

And it isn't only the Republican nominee who has been burnt by the sharper glare of the spotlight on their national security credentials as they assume their party's mantle.

Amidst the denunciation of Trump's statements Wednesday, his campaign pointed to Clinton's use of personal server for official business as secretary of state despite handling sensitive classified information in that role and the thousands of emails she didn't hand over to the State Department while destroying the server itself -- the very emails Trump was referring to. Senior policy adviser Stephen Miller blasting "Hillary Clinton's enablement of foreign espionage with her illegal email scheme."

FBI chief James Comey criticized Clinton's use of a private email server, though he recommended that no criminal charges be brought against her.

Trump's remarks about Russia and Clinton's emails came at a news conference in which he was discussing the hack of the Democratic National Committee that led to the publication of thousands of emails on WikiLeaks. The emails showed officials strategizing against Clinton primary rival Bernie Sanders in his campaign for the Democratic nomination at a time when the DNC claimed it was neutral in the race.

US officials believe that the hacking likely originated in Russia, opening the possibility that it was the result of an espionage operation. The Clinton campaign alleges that the leak was a deliberate attempt to damage her campaign and help Trump, who some experts believe the Kremlin prefers to be the next US president.

But James Clapper, the director of the Office of National Intelligence, urged caution Thursday, telling an audience at the Aspen Security Forum, "I don't think we are quite ready yet to make a call on attribution."

He indicated that the response to the incident had been somewhat overblown, describing it as "hyperventilation."

Trump's campaign has dismissed the idea that the Kremlin is trying to help the real estate mogul win and was quick to say Wednesday that the GOP nominee wasn't "inviting" an attack by Russia's highly capable intelligence agencies on the US government or private networks in the United States.

Vice presidential nominee Mike Pence came to Trump's defense Thursday, accusing reporters and pundits in Philadelphia of taking his remarks out of context.

"They've taken a sarcastic comment, suggested that he was encouraging that activity all the while ignoring the extraordinary revelations in these emails of collusion of horrible statements regarding race, ethnicity and religion," Pence said on Laura Ingraham's radio show.

Trump's campaign may be discounting the furor, but it is undeniable that his comments on Wednesday -- in which he also appeared to open the way to recognizing Russia's annexation of the Ukrainian territory of Crimea in contravention of current US policy -- offered an opening for the Clinton campaign.

"His fascination and admiration for Vladimir Putin is quite peculiar. He has taken policy positions that are clearly seeing to advance Russia's interests against the interests of our European allies," Clinton campaign spokesman Brian Fallon told CNN.

"On issue after issue, he has shown a strange affinity for Vladimir Putin and Putin's Russia."

The lack of open public support from senior Republican officials during the controversy only widened the Clinton campaign's opportunity.

Speaker after speaker at the Democratic convention in Philadelphia on Wednesday lambasted Trump over his suitability to be president.

"Donald Trump says he gets his foreign policy experience from watching TV and running the Miss Universe pageant. If only it were funny. It is deadly serious," said former CIA Director Leon Panetta. "Donald Trump is asking one of our adversaries to engage in hacking or intelligence efforts against the United States to affect our election."

Retired Navy Adm. John Hutson also piled on.

"This morning, this very morning, (Trump) invited Russia to hack us. That's not law and order, that's criminal intent," he said, playing off a slogan that Trump emphasized at the Republican convention last week.

And in his marquee speech concluding the evening, Obama slammed Trump for cozying up to Putin. The billionaire businessman has offered a string of favorable remarks about the Russian strongman's leadership style by the billionaire -- part of a remarkable scenario in which Democrats, often blasted as weak during the Cold War by the GOP are now emerging as Kremlin-baiters in trying to argue that the Republican nominee is unfit to be president.

Were he not so vehemently opposed to Trump, Obama could give the billionaire a lesson in how a single remark or offhand equip on issues of national security can haunt a president for years.

Commanders in chief and other senior members of the national security establishment speak so carefully, they often resort to the cliches of diplomatic speak in the knowledge that what they say immediately rockets round the world.

Obama is still suffering the consequences of saying in a White House news conference in 2012 that Syria's use of chemical weapons would be a "red line" for him.

Obama's decision not to attack the regime of Syrian President Bashar al-Assad for its use of such arms was seen by critics as a disastrous blow to not just his own credibility but that of the United States itself.

The fallout from the episode raised questions about whether the US word on its security guarantees elsewhere -- for example with treaty allies in Europe and Asia -- was as absolute as many nations had assumed.

Trump's comments during the campaign about NATO and musings on whether US allies Japan and South Korea should consider nuclear weapons have similarly concerned those countries. Though just a candidate, the policies would have major implications for their security should he be the next occupant, causing consternation in foreign ministries around the world.

Another brief comment on national security that lingered for years occurred when an unnamed senior official described the US support role in Libya in the New Yorker as "leading from behind."

Ever since, the phrase has incessantly been used by Republicans to argue that Obama is a weak leader.

In another New Yorker piece, Obama famously referred to ISIS as the "JV" team -- a comment that has plagued him since the group seized territory to create a self-declared caliphate in the Middle East and started staging and inspiring terror attacks in Europe and the US.

And Obama is far from the first American president to discover how unwise remarks on national security can hurt.

His predecessor, George W. Bush, for instance, found that early in his presidency that imprecise language could trip him up, as he stumbled into controversies on Taiwan policy and the terminology used to refer to terrorism after 9/11.

And sometimes, presidential sarcasm can simply lead to commanders in chief being accused of acting in bad taste.

In 1984, while the standoff with the Soviets still raged, President Ronald Reagan quipped during an off-air sound check that he had "signed legislation that will outlaw Russia forever. We begin bombing in five minutes."

The comment prompted then-Democratic nominee Walter Mondale to remark: "A president has to be very, very careful with his words."