WASHINGTON — Former Dallas mayor and congressman Steve Bartlett has joined a growing list of Republicans to rebuke GOP presidential nominee Donald Trump, signing a letter that says the billionaire "makes a mockery of the principles and values we have cherished."

Bartlett, who served in Congress in the 1980s and as mayor in the early 1990s, signed the missive as part of a group of 30 former Republican members of Congress. Since the signatories are listed alphabetically, Bartlett's name appears at the top of the list.

The letter released Thursday says a president must have the "competence, intelligence, knowledge, understanding, empathy, judgment and temperament" to keep America on course. Trump "fails on each of those measures," the letter says.

"We urge our fellow Republicans not to vote for this man whose disgraceful candidacy is indefensible," the letter says.

The Trump campaign didn't immediately respond to a request for comment.

Steve Bartlett, a former Dallas mayor and congressman, has joined a growing list of Republicans to bash Donald Trump. (Daniel Rosenbaum / The New York Times)

Bartlett, influential in the business community, is far from the only Republican to express his or her discomfort with the real estate mogul.

Several other groups of former GOP lawmakers, appointees and staffers have publicly voiced their opposition to Trump. Even former President George H.W. Bush reportedly shared at a private gathering last month that he would be voting for Democrat Hillary Clinton.

Bartlett also isn't the only Dallas mayor to get involved in the 2016 race.

Current Dallas Mayor Mike Rawlings, a Democrat, is a "Senator Clinton fan" who recently participated in a Big D fundraiser for her campaign. But former Dallas Mayor Tom Leppert, a Republican, backed Trump over even some fellow Texans in the GOP presidential primary.

Though the letter by Bartlett and his colleagues urges Republicans to ditch Trump, it doesn't encourage them to vote for Clinton.

But it dings Trump for his comments about women, the handicapped and war heroes, among others. It says Trump's "public statements are peppered with lies." And it says Trump is an "unacceptable danger" who is "manifestly unqualified to be president."

"We may differ on how we will cast our ballots in November but none of us will vote for Donald Trump," the letter says.

To read all the signatories, please click here.