By Chandelis Duster, CNN

(CNN) -- More than two dozen former Republican lawmakers announced Monday they are endorsing Joe Biden for president.

Former Sen. Jeff Flake of Arizona and former Rep. Charlie Dent of Pennsylvania are among those throwing their support behind the Democratic presidential nominee through "Republicans for Biden," and the endorsements come on the morning of the first day of the Republican National Convention.

Biden has repeatedly emphasized Republican support as he looks to build a broad coalition in his campaign against President Donald Trump.

While the endorsements offer a symbolic boost to Biden as he seeks to win over persuadable voters, Trump is still overwhelmingly popular among Republicans, a point made by Trump campaign spokesman Tim Murtaugh, who dismissed the significance of the endorsements.

"Joe Biden has been a failure in the Washington Swamp for a half century, so no one should be surprised when Swamp creatures gather to protect one of their own," Murtaugh said. "President Trump has unprecedented support — over 95 percent — among real Republican voters and is also making strong inroads in Biden's core Democrat constituencies, like Black Americans, Latinos, and union members. President Trump's record of success for all Americans will carry him to victory in November."

Along with Flake and Dent, former Sens. Gordon Humphrey of New Hampshire and John Warner of Virginia added their support, as did the following former members of Congress:

Texas Rep. Steve Bartlett

Pennsylvania Rep. Bill Clinger

Missouri Rep. Tom Coleman

Hawaii Rep. Charles Djou

Oklahoma Rep. Mickey Edwards

Maryland Rep. Wayne Gilchrest

Pennsylvania Rep. Jim Greenwood

South Carolina Rep. Bob Inglis

Arizona Rep. Jim Kolbe

California Rep. Steve Kuykendall

Illinois Rep. Ray LaHood

Iowa Rep. Jim Leach

New York Rep. Susan Molinari

Maryland Rep. Connie Morella

Mississippi Rep. Mike Parker

New York Rep. Jack Quinn

Rhode Island Rep. Claudine Schneider

Connecticut Rep. Christopher Shays

Vermont Rep. Peter Smith

Texas Rep. Alan Steelman

New York Rep. Jim Walsh

Virginia Rep. Bill Whitehurst

New Jersey Rep. Dick Zimmer

Dent, who announced last week he was endorsing Biden, told CNN's John Berman on "New Day" Monday there is concern among Republicans that the party is becoming "Trumpian" and driven by the power of one man.

"There are a lot of former Republican members of Congress who are unhappy with the direction of the party," he said when asked what was behind the coalition. "We understand that political parties are not static, they're dynamic. But when they change, they should change for the better. And I think it's fair to say that they are concerned about this party becoming so Trumpian, frankly, driven by the power of one man. I think there's a lot of concern about the nativism, the protectionism, the isolationism. These are very concerning attributes."

He also said Trump's endorsement of Marjorie Taylor Greene, a Republican congressional candidate in Georgia with a history of promoting QAnon theories and making racist remarks, has some concerned "the leader of our party is embracing some, you know, pretty extreme type."

For Flake, backing Biden is the latest salvo in a long-running feud with the President. An outspoken critic of President, he previously announced he would not vote for Trump for reelection and in April said a "sound defeat" would be better for the Republican Party. Trump has slammed the former senator, calling the senator's career "toast" in 2017 after Flake was overheard at an event criticizing the President and then-Alabama Republican Senate candidate Roy Moore.

A number of prominent Republicans have come out in support of Biden in recent weeks, including former Ohio Gov. John Kasich and former Secretary of State Colin Powell, who both spoke at the Democratic National Convention last week.

This story has been updated.