“My marriage to Chasten has made me a better man and yes, Mr. Vice President, it has moved me closer to God . . . If me being gay was a choice, it was a choice that was made far, far above my pay grade.
“And that’s the thing I wish the Mike Pences of the world would understand—that if you’ve got a problem with who I am, your problem is not with me. Your quarrel, sir, is with my creator”.
He held his launch event in South Bend, the Indiana town where he has served as mayor since 2012.
Setting out his vision to “re-imagine our future”, he promised a “totally different” politics from Mr Trump.
Should he win, Mr Buttigieg would be the first openly gay US president.
At 37, he is the youngest entrant in a crowded field of Democratic candidates that features Vermont Senator Bernie Sanders, 77, and 69-year-old Massachusetts Senator Elizabeth Warren.
Despite his relative inexperience, the former Rhodes Scholar has surged in the polls in recent weeks, propelling him from the margins to the front of the pack.
In a veiled attack on the Trump administration, he railed against the “myth being sold” by those who sought to “stop the clock and turn it back”.
“It is time to walk away from the politics of the past and toward something totally different,” he told supporters at his launch event.
His campaign has so far focused on gun control, climate change and LGBT rights and other progressive issues he hopes will appeal to millennial voters.
Mr Buttigieg’s track record as mayor of South Bend, which has seen a struggling economy improve during his tenure, has also earned him plaudits.