"We are proud to endorse Hillary Clinton for president, and believe that she is the champion we can count on in November — and every day she occupies the Oval Office," he added.
Clinton will officially accept the endorsement at an event in Des Moines, Iowa on Sunday with Griffin, the organization said.
In a statement Tuesday, Clinton pledged to support the Equality Act, which would amend the Civil Rights Act of 1964 to include discrimination against LGBT people, she would support transgender people serving in the military, and would end the practice of gay 'conversion therapy' on minors.
“I'm honored to receive the endorsement of the Human Rights Campaign—the nation’s largest organization working to achieve full equality for lesbian, gay, bisexual, and transgender Americans,: Clinton said in a statement. "Thanks to the Human Rights Campaign and millions of advocates across the country, we’ve made tremendous progress. But our work is far from over. Too many LGBT Americans still face discrimination—in employment, in housing, in education, in health care—because of who they are or who they love. And the stakes in this election couldn't be higher. The Republican candidates for president have not only hurled hateful, insulting rhetoric about the LGBT community—they’ve made it clear that if elected, they will roll back the rights that so many have fought for.
Too many LGBT Americans still face discrimination—in employment, in housing, in education, in health care—because of who they are or who they love. And the stakes in this election couldn't be higher," she added.
In 2013, Clinton taped a message of support for same-sex marriage on behalf of HRC, though the announcement raised questions about the timing of her evolution on the issue. Clinton also has credited her daughter, Chelsea, with encouraging her to speak out in support of same-sex marriage while she was secretary of state.
In her message in support of gay marriage, Clinton noted that her daughter's recent marriage also contributed.
"A few years ago, Bill and I celebrated as our own daughter married the love of her life, and I wish every parent the same joy," Clinton said. "To deny that opportunity to any of our daughters and sons solely on the basis of who they are and who they love is to deny them the chance to live up to their own God-given potential."
HRC endorsed Barack Obama in 2008 and 2012, though Obama did not publicly come out in support of gay marriage until 2012.
Clinton also has been questioned about the role that former president Bill Clinton played in signing the Defense of Marriage Act. She noted at the time that DOMA was seen as the lesser evil when a constitutional amendment banning gay marriage was being considered in "private discussions."
Clinton has made LGBT rights one of her campaign's pillars. In an October speech for HRC, Clinton warned that Republicans would try to roll back marriage equality and gay rights, and she pledged she would defend them as president.
"I’ve been fighting alongside you and others for equal rights and I’m just getting warmed up," Clinton said. "That’s a promise, from one HRC to another."