WASHINGTON — Nearly two dozen senators will ask Defense Secretary Ash Carter on Thursday to take action to include explicit nondiscrimination protections for lesbian, gay, and bisexual service members — calling inaction on the issue since the repeal of "don't ask, don't tell" a "failure."

Led by Democratic Sens. Chris Murphy and Tammy Baldwin, the 23 senators, in a copy of the letter provided to BuzzFeed News, "strongly urge" Carter to update equal employment opportunity policies at the Pentagon and in all the service branches "to prevent discrimination, harassment, or intimidation of service members based on sexual orientation." The group includes Republican Sen. Susan Collins, a leader on DADT repeal in 2010.

The question of whether sexual orientation would be added to equal employment opportunity policies in the military was raised by reporters and advocates even before the repeal of DADT took effect in September 2011. In Thursday's letter, the senators write to Carter, however, "equal opportunity policies for the military continue to lag behind" policies for the military's civilian employees.

"The repeal of DADT represented great progress toward eradicating a significant barrier to formal equality, but the military is not yet an equitable environment for gay, lesbian, and bisexual service members," the senators write. "The absence of formal equal opportunity protections not only undermines foundational American principles of fairness and equality, it also presents an unneeded risk to national security by negatively impacting the morale and readiness of our all-volunteer force."

Notably, the letter makes no mention of the continued regulatory obstacles to out transgender service in the military — another key area where LGBT advocates have been pressing for military action in the second term of the Obama administration.

Asked about the decision not to mention transgender service in the letter, a spokesman for Murphy, Chris Harris, told BuzzFeed News, "This letter is an effort to close the book on the repeal of 'Don't Ask, Don't Tell' by urging the Pentagon to finally update its equal opportunity policies and ban discrimination on the basis of sexual orientation. Expanding rights and opportunities for transgender Americans seeking to serve their country is hugely important, but is a separate issue from fully implementing the repeal of DADT as passed into law in 2010."

In addition to Murphy, Baldwin, and Collins, the letter also was signed by Sens. Richard Blumenthal, Cory Booker, Barbara Boxer, Maria Cantwell, Ben Cardin, Chris Coons, Dick Durbin, Dianne Feinstein, Martin Heinrich, Mazie Hirono, Al Franken, Kirsten Gillibrand, Amy Klobuchar, Bob Menendez, Barbara Mikulski, Gary Peters, Bernie Sanders, Brian Schatz, Jeanne Shaheen, and Elizabeth Warren.