During her speech at the Democratic National Convention, Clinton criticized Trump's use of Twitter as a reason enough not to elect him. "Imagine him in the Oval Office facing a real crisis," she said. "A man you can bait with a tweet is not a man we can trust with nuclear weapons."
Her campaign also put out an ad in early August titled, "Unfit."
A recent poll found that 46% of registered voters believe that Trump would "authorize the use of a nuclear weapon."
But what has he actually said on the matter?
"I will never rule it out."
In late March, in an interview with MSNBC's Chris Matthews, Trump made it clear that using nuclear weapons would be seen as a last resort. But he refused to take the option off the table, saying it would make him a bad negotiator.
Matthews asked Trump if he would use nuclear weapons against Western European allies. Trump said he didn't want to, but added, "I am not taking cards off the table."
"I will be the last to use nuclear weapons. It's a horror to use nuclear weapons. The power of weaponry today is the single greatest problem that our world has," he said. "I will be the last to use it. I will not be a happy trigger like some people might be. I will be the last, but I will never ever rule it out."
"You want to be unpredictable."
In addition, Trump touted his ability to successfully negotiate deals by being unpredictable. "Somebody recently said ?— ?I made a great business deal," he said. "And the person on the other side was interviewed by a newspaper. And how did Trump do this? And they said, he's so unpredictable."
"They don't even know if they work."
It's that reason Trump has said other countries should build a nuclear arsenal, including Japan and South Korea, in case the United States is unable to protect them from threats from China or North Korea.
"We're not gonna have to nuke anybody."
Trump has said he is so confident that he will build such a strong U.S. military that there will be no need for nuclear weapons.