Tampa Mayor Bob Buckhorn on Thursday endorsed Gwen Graham in the Democratic primary for governor, giving the former Congresswoman a high-profile backer in the crucial Interstate-4 corridor.

"I believe in her, because I believe she is the right candidate at the right time for this point in Florida's history," Buckhorn said. "Twenty years of one-party control is two much. We need a voice of reason."

Asked after why he chose Graham over her competitors, Buckhorn said simply: "Gwen can win."

Buckhorn, a twice-elected mayor, is one of the region's most popular Democrats for leading a post-recession Tampa toward an economic renaissance. The ongoing revitalization of Tampa's downtown, the construction of the Riverwalk and the opening of the new Julian B. Lane Riverfront Park have all occurred under Buckhorn's watch.

Graham said Tampa's story — which Buckhorn so often likes to tell — is a  "success story we can take all across Florida."

Buckhorn's endorsement of Graham at the iconic Columbia Restaurant in Ybor City also comes amid much speculation that Buckhorn could join the Aug. 28 winner on the ticket as candidate for lieutenant governor.

Graham is running in the Democratic primary against Tallahassee Mayor Andrew Gillum, former Miami Beach Mayor Philip Levine, Palm Beach real estate mogul Jeff Greene and Winter Park businessman Chris King.

For months, Buckhorn has been rumored to be a potential running mate for Graham. His pro-business and pragmatic politics share Graham's centrist leanings. He can tout progressive accomplishments like paid family leave but he also played nice with Republicans in Tallahassee when it benefited the city. Graham, too, has demonstrated a willingness to cross the aisle when needed.

Buckhorn said he wasn't auditioning for the job but would consider it if asked — whoever Democratic voters pick in 12 days, not just Graham.

"I don't know that they would," Buckhorn added.

Although Buckhorn made his endorsement official Thursday, he came out in support of Graham in July, when Graham was getting hammered with negative ads paid for by a secret money group backing opponent Andrew Gillum.

If he is the choice, Buckhorn will be expected to help fundraise for a Democrat who is likely to go up against a well-funded Republican.

Buckhorn's record on raising big dollars is mixed. He has cozied up to local deep-pocketed Democratic benefactors for Hillary Clinton, but his own political action committee, One Florida, never got off the ground and raised just $140,000.

On Thursday, Graham said it was too soon to talk about Buckhorn's prospects as a running-mate. After the event, however, she said what she was looking for in a lieutenant governor.

"My number one criteria is somebody who can help me govern and be my friend and my partner every single day," she said.

Buckhorn, 60, was once considered a contender for the governor's mansion himself. Never was the buzz greater than at the 2016 Democratic National Convention in Philadelphia, when Buckhorn drew rave reviews after a rousing speech to the Florida delegation. The appearance upstaged Graham, whose TelePrompTer-aided delivery fell flat.

However, Buckhorn ultimately decided against a run for governor, choosing instead to finish his second term for mayor and stay close to his two daughters.

His daughters, in fact, were an inspiration for his endorsement of Graham, who if elected would be Florida's first female governor.

"As the father of two little girls, I love breaking those ceilings," Buckhorn said. "I love being able to tell my daughters that I participated in making history and did it for the right reasons."

Times/Herald staff writer Lawrence Mower contributed to this report.