The Democratic presidential front-runner unleashed her sharpest attacks yet on Trump, the presumptive GOP nominee, in an exclusive interview with CNN's Chris Cuomo in Park Ridge, Illinois.
She pointed to Trump's attacks on British politicians, his willingness to speak with North Korean dictator Kim Jong Un, his call for the United States to back away from the North Atlantic Treaty Organization and his assertion that more countries should have nuclear weapons, and said it "adds up to a very troubling picture."
"I know how hard this job is, and I know that we need steadiness as well as strength and smarts in it, and I have concluded he is not qualified to be president of the United States," Clinton said.
When asked whether Trump was qualified in April, Clinton told CNN's Jake Tapper
, "Well, the voters will have to decide. I'm going to lay out my qualifications to be president."
On Thursday, Clinton was particularly critical of Trump's proposal to ban Muslims from entering the United States, calling the idea "provocative" and saying it sends a "message of disrespect" to important majority-Muslim nations that are U.S. allies.
"When you run for president of the United States, the entire world is listening and watching," she said. "So when you say you're going to bar all Muslims, you're sending evidence to the Muslim world, and you're also sending a message to terrorists. ... Donald Trump is essentially being used as a recruiter for more people to join the cause of terrorism."
Trump later in the afternoon shot back in a statement
, saying, "The fact that Hillary thinks the temporary Muslim ban, which she calls the "Muslim ban", promotes terrorism, proves Bernie Sanders was correct when he said she is not qualified to be President."
"Ask Hillary who blew up the plane last night -- another terrible, but preventable tragedy," the statement added. "She has bad judgment and is unfit to serve as president at this delicate and difficult time in our country's history."
Clinton said she's not going to respond to Trump's attacks on her as an "enabler" of her husband Bill Clinton's marital infidelities and treatment of women. "I know that's exactly what he's fishing for, and I'm not going to be responding," she said.
Instead, she said, she plans to focus her attacks on Trump on "what he says about other people" -- a cue the pro-Clinton Priorities USA super PAC took in launching its first anti-Trump ads, highlighting insults he has hurled at women.
"I'm going after him exactly on those issues and statements that are divisive and dangerous, and I actually think that's what the American people want to see," Clinton said.
Clinton also addressed the disappearance of EgyptAir Flight 804,
saying the disaster "shines a very bright light on the threat that we face from organized terror groups."
"It reinforces the need for American leadership -- the kind of hard, steady leadership that only America can provide," Clinton said.
EgyptAir Flight 804 disappeared early Thursday morning with 66 people on board above the waters of the Mediterranean Sea and searches have found the plane's wreckage.
Clinton's tense fight with Sanders
The Democratic presidential front-runner said that she will be the party's nominee, because she's got an "insurmountable" number of pledged delegates and has received millions more votes overall.
"I will be the nominee for my party, Chris. That is already done, in effect," she said. "There is no way that I won't be."
Clinton wouldn't say whether Sanders was being considered for her running mate and said the Vermont senator needs to "do his part" to unify the party going into November.
She highlighted her role in unifying Democrats -- including the 40% of Clinton supporters who had said they wouldn't support Barack Obama if he won the party's nomination -- in 2008, the last close Democratic nominating contest.
"That's why the lesson of 2008 -- which was a hard-fought primary, if you remember -- is so pertinent here. Because I did my part, but so did (then-)Sen. Obama," she said. "We went to Unity, New Hampshire, together, appeared together, spoke together, and made it absolutely obvious that I was supporting him, that he was grateful for that support."
Sanders' campaign responded to Clinton in a statement later Thursday afternoon.
"In the past three weeks, voters in Indiana, West Virginia and Oregon respectfully disagreed with Secretary Clinton," the campaign said. "We expect voters in the remaining eight contests also will disagree. And with almost every national and state poll showing Sen. Sanders doing much, much better than Secretary Clinton against Donald Trump, it is clear that millions of Americans have growing doubts about the Clinton campaign."