Fifty former national security officials from Republican administrations have signed a letter opposing party presidential nominee Donald Trump, calling him an unqualified choice who would put the nation at risk.

"None of us will vote for Donald Trump," the officials wrote. "Most fundamentally, Mr. Trump lacks the character, values, and experience to be President."

The co-signers include security and foreign policy officials who served in Republicans administrations dating back to the Richard Nixon administration. It also includes officials who served more recently, under George W. Bush, like ex-Homeland Security Secretary Michael Chertoff.

The letter was released as another Republican lawmaker -- Maine Sen. Susan Collins -- wrote an op-ed claiming she would not support Trump.

But Trump, in a statement, dismissed the latest letter signed by what he called "insiders."

"The names on this letter are the ones the American people should look to for answers on why the world is a mess, and we thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place," he wrote.