It was the Democratic nominee's most forceful denunciation after Trump caused a firestorm by suggesting to supporters in North Carolina on Tuesday that "Second Amendment people" -- those who support gun rights -- could take action to stop Clinton from appointing US Supreme Court justices as president.
"Yesterday we witnessed the latest in a long line of casual comments by Donald Trump that crossed the line," Clinton told a rally in Des Moines, Iowa.
"Words matter, my friends. And if you are running to be president, or you are president of the United States, words can have tremendous consequences."
Trump and his campaign had quickly sought to douse the flames, insisting the Republican flagbearer was merely urging gun rights supporters to reject her candidacy at the ballot box.
Clinton appeared to reject the Trump campaign's defense, warning of the dangers of reckless language during a presidential race.
She slammed Trump's "casual cruelty to a gold star family," referring to the billionaire's clash with the parents of a Muslim American soldier who was killed in action.
"His casual suggestion that more countries should have nuclear weapons, and now his casual inciting of violence," she added.
"Every single one of these incidents shows us that Donald Trump simply does not have the temperament to be president and commander in chief of the United States."
RELATED READING
Nine times when Trump said he didn't say what he said
With Team Trump seeking to dig the candidate out of a deepening hole, former New York mayor and Trump backer Rudy Giuliani insisted the uproar was triggered by "the Clinton spin machine."
But the Secret Service, tasked with protecting the president and presidential nominees, was taking Trump's remarks seriously and has spoken with Trump's campaign about them, CNN reported.
In an unnerving example of campaign security tensions, an animal rights activist appeared to rush the stage as Clinton spoke Wednesday, but was tackled and removed by security.
Secret Service agents jumped up to protect the candidate, then retreated after the protester was escorted out.
RELATED READING
Trump suggests gun rights supporters could stop Clinton
'Under siege'
Clinton meanwhile launched an open appeal Wednesday to independents and Republicans repulsed by Trump over his string of controversial statements.
The campaign unveiled a new website, togetherforamerica.com, that lists dozens of Republicans and independents who back Clinton, including former director of national intelligence John Negroponte and former New York mayor Michael Bloomberg.
"Regardless of party, voters are increasingly concerned that Trump's tendency to bully, demean and degrade others sends the wrong message to our children," Clinton campaign chairman John Podesta said in announcing the effort.
Lawmakers, former national security officials and other critics expressed concern that Trump had advocated violence, possibly in jest, against Clinton or her Supreme Court nominees.
"If she gets to pick her judges, nothing you can do, folks," Trump, 70, said Tuesday. "Although the Second Amendment people -- maybe there is, I don't know."
Trump on Wednesday steered clear of the controversy at a Virginia rally, although he reiterated that the Second Amendment remained "under siege."
Trump has suffered what critics insist is a long string of missteps that have marred his campaign since he officially won the nomination last month, prompting several Republicans to reject his candidacy.
He has clearly roiled the party with his unorthodox remarks, with some Republicans frustrated at his apparent inability to stay on message.
A Reuters/IPSOS poll Wednesday found that 19 percent of Republican voters want the real estate tycoon to drop out of the race, while 70 percent think he should stay and 10 percent say they don't know.
The RealClearPolitics national poll average shows Clinton leading Trump by 48 percent to 40 percent.
Fifty prominent Republican national security experts announced in an open letter this week they would not vote for Trump, saying he "lacks the character, values and experience" to be president.
Six GOP senators including Susan Collins and a number of House Republicans have disowned him too.
Emails, again
Meanwhile, Clinton was enduring a fresh round of criticism over her emails from her time as secretary of state, which have been a thorn in her side and hurt her trustworthiness among voters.
Conservative watchdog Judicial Watch released a batch of emails that raise questions about the State Department's relationship with the Clinton Foundation.
The Trump campaign seized on the latest emails to blast Clinton as "corrupt," with critics saying the messages showed the foundation sought improper preferential treatment from the department.
