Paul Selva, once one of Donald Trump's top generals, endorses Joe Biden

Once one of President Donald Trump's most senior military advisers, retired General Paul Selva is joining a large group of former Pentagon leaders to publicly endorse Democratic candidate Joe Biden for the Nov. 3 presidential election, according to a letter seen by Reuters.

Selva, who served as vice chairman of the Joint Chiefs of Staff until July 2019, appears on a list of 489 national security experts - including former military leaders, ambassadors and White House officials - who signed a letter being released on Thursday that declares Trump "not equal to the enormous responsibilities of his office."

"Thanks to his disdainful attitude and his failures, our allies no longer trust or respect us, and our enemies no longer fear us," reads the letter by the group, called "National Security Leaders For Biden."

Selva could not be immediately reached for comment.

Other groups of former national security leaders have endorsed Biden and criticized Trump, but it is remarkable that a recently retired four-star general like Selva -- who was the Pentagon's No. 2 military officer -- would publicly endorse any candidate and sign onto a letter condemning a president he served.

Beyond Selva, retired Admiral Paul Zukunft, who served as commandant of the Coast Guard from 2014 until 2018, also appears on the long list of signatories.

Like Zukunft, Selva was not chosen for his position by Trump, who inherited military leaders picked by his Democratic predecessor, Barack Obama.

Other prominent retired military officers endorsing Biden in the letter include: Samuel Locklear, a retired Navy admiral who led the U.S. Pacific Command until 2015, and Peter Chiarelli, who retired in 2012 after serving as the Army's vice chief of staff.

Biden's campaign has been trying to build a broad coalition of liberals, moderate Republicans and independents. When he accepted the Democratic nomination at his party's convention in August, Biden said if elected he would be a president for all Americans, not just for the Democratic base.

Although Trump describes himself as a champion of the U.S. military and boasts of hefty military spending, his relationship with the Pentagon has been strained. He has ridiculed top generals, ignored their advice on some key issues and questioned their commitment to their soldiers.

Earlier this month, Trump sought to portray himself a champion of the rank-and-file soldier.

"I'm not saying the military's in love with me. The soldiers are," Trump told reporters. "The top people in the Pentagon probably aren’t because they want to do nothing but fight wars so all of those wonderful companies that make the bombs and make the planes and make everything else stay happy."