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"He's not a serious adult," Sasse said in the ad

The ad closes with the words: "Unfit. Dangerous. Even for Republicans."

Washington CNN  — 

A new Hillary Clinton ad is turning to Republicans to make the case that Donald Trump is unfit for the Oval Office.

The ad, titled “Agree,” features clips of GOP leaders criticizing the business mogul in interviews and on news programs.

Criticisms come from 2012 Republican presidential nominee Mitt Romney as well as longtime Trump critics such as South Carolina Sen. Lindsey Graham, Nebraska Sen. Ben Sasse and Arizona Sen. Jeff Flake.

“He’s not a serious adult,” Sasse says in an interview he gave on MSNBC.

The ad closes with the words: “Unfit. Dangerous. Even for Republicans.”

Trump lashed back at the Republicans in the video late Saturday morning.

“Really sad that Republicans would allow themselves to be used in a Clinton ad. Lindsey Graham, Romney, Flake, Sass. SUPREME COURT, REMEMBER!” he tweeted.

The spot is part of an existing ad buy in the Florida, Iowa, North Carolina, Nevada, Ohio, Pennsylvania and Omaha, Nebraska, media markets. It is also part of a national cable advertising campaign.

The ad was released a day after Clinton touted the support she had received from national security leaders in a brief press conference.

“No conversation about our national security would be complete unless we acknowledged that the nominee on the other side promises to do things that will make us less safe,” Clinton told reporters. “National security experts on both sides of the aisle are chilled by what they’re hearing from the Republican nominee.”

Clinton has recently reached out to Republicans wary of Trump. In a statement announcing the ad Saturday, the campaign wrote, “As senator and as secretary of state, Hillary Clinton demonstrated her ability to work across the aisle to get things done.”

The ad’s release on Saturday, however, came in the wake of her remarks grouping half of Trump’s supporters into a “basket of deplorables,” a statement for which she has been widely criticized.

Clinton surrogates have emphasized that Clinton was talking about Trump’s supporters – people who attend his rallies – and not Trump voters, a larger group, in the eyes of the campaign, that includes more moderate Republicans.

CNN’s Dan Merica contributed to this report.