Kirk, who returned to the Capitol recently after suffering a debilitating stroke, expressed his views on same-sex marriage a week after Ohio Republican Sen. Rob Portman, who was on the short list to serve as Mitt Romney's running mate in the 2012 election, announced he had changed his mind after his son, Will, came out as gay.
USA Today said Kirk has spoken openly about the transformative experience he's had since recovering from the stroke. He had to learn how to walk again.
"Our time on this Earth is limited, I know that better than most," Kirk said in a statement. "Life comes down to who you and love and who loves you back -- government has no place in the middle."
Kirk doesn't face re-election in deep-blue Illinois until 2016.
As for Carper, he became just the latest in a torrent of Democrats to endorse gay marriage after President Barack Obama made his support official last year -- after Vice President Joe Biden beat him to the punch.
Carper told The Washington Post he applied the "golden rule" to the question.
"It calls on us to treat others as we want to be treated," Carper said. "That means, to me, that all Americans ultimately should be free to marry the people they love and intend to share their lives with, regardless of their sexual orientation, and that's why today, after a great deal of soul searching, I'm endorsing marriage equality."
There are now just seven Democratic senators -- all hailing from states that lean Republican -- who have yet to endorse same-sex marriage. They are: Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota, Tim Johnson of South Dakota, Mary Landrieu of Louisiana, Joe Manchin of West Virginia, Bill Nelson of Florida and Mark Pryor of Arkansas.