TAMPA -— Everything looks to be breaking Jane Castor's way in the race for Tampa mayor.

She came within a whisker of winning the office outright in Tuesday night's primary, and got a big endorsement Wednesday morning from outgoing mayor Bob Buckhorn.

"She is prepared to be our mayor. She has trained for this moment," Buckhorn said of Castor, the former Tampa police chief who received three times as many votes as the runner-up, retired banker David Straz.

The two are now headed for an April 23 runoff. Straz, a multi-millionaire, said Wednesday he has no intention of dropping out. He promised instead to spend whatever it takes to win.

At an afternoon news conference, he said Buckhorn's endorsement of Castor is hardly surprising.

"The cabal that exists, the good old boy network that exists in the city. When you put all the facts and figures together, you would expect that to happen," Straz said, adding that he has "great respect" for Buckhorn.

Related: Jane Castor will face David Straz in Tampa mayor's race

On its first day, at least, the seven-week runoff election was a battle of competing visions set in graceful waterfront parks.

Buckhorn endorsed Castor in Water Works Park, opened in 2014 at a cost of $7.4 million in Tampa Heights. Straz met with reporters in the $35.5 million Julian B. Lane Park that opened in May.

Buckhorn and Castor stood near the mayor's signature piece of real estate, the Riverwalk, while speaking brightly of downtown Tampa's transformation in recent years.

Straz stood in a pristine 25-acre park and talked about needs in other neighborhoods. He said the vast green space Buckhorn calls Tampa's version of Central Park was a mistake.

"This $35 million park, which is quite nice, has been built... while other neighborhoods in Tampa have been neglected," he said. "I would share the prosperity with all of our neighborhoods."

The runoff will be an uphill battle for Straz, who received just 15.5 percent of the vote Tuesday in the primary that winnowed seven mayoral candidates to two. Castor got 48 percent of the vote, or just 980 shy of the 50 percent plus one that would have have won her the election outright.

Buckhorn, who is leaving office because of term limits, said Castor, 59, is the only candidate in the race who can continue the path he has set for the city.

"I saw her tested over and over and over again. And she always rose to the occasion," Buckhorn told reporters as he stood next to his former police chief. "There is only one person in this race who is equipped, who is prepared, who is ready to be the mayor."

Castor said Buckhorn has started a renaissance she intends to continue.

"We've got to put our foot on the gas and keep going," she said.

Asked whether Straz should withdraw from the non-partisan race given the results Tuesday night, Buckhorn said, "That's only a decision Mr. Straz can make."

Straz wasted no time making it.

"I will not consider conceding under any circumstances because the people of Tampa deserve having the best as their mayor. And that's me," he said.

The 76-year-old also pledged to keep dipping into his personal fortune, pegged at $426-million in his financial disclosure.

So far, he's spent nearly $3 million — more than double the previous record for a mayoral race set in 2003 by Frank Sanchez, who lost to Pam Iorio.

The money doesn't concern him, he said.

"The people that don't like spending that much money are the people who don't have it. But I'm happy to spend it in my love for the city of Tampa," Straz said.

The two campaigns had different reactions to questions about whether negative campaign ads in the final days of the primary would continue in the runoff.

"People in the community want to be inspired and not divided,'' Buckhorn said. "If you have a plan run on it. Tearing down an opponent is not a strategy to win."

Castor said she intends to remain positive.

"People here in Tampa want a leader," she said.

Straz said he would go negative as long as it was "honest and truthful."

"This is a free country, right? You can do whatever you want to do. I plan to run a campaign that's truthful and honest. If there are things that come up that are negative as long as they're truthful and honest, I think that's perfectly alright," he said.

Castor said she expected to receive the support of candidates who were eliminated Tuesday. So far, only one has signed on. Retired judge Dick Greco Jr. told the Tampa Bay Times on Wednesday that he called Castor Tuesday night to congratulate her and offer his support for the runoff. Greco received 4,156 votes equaling 8.55 percent.

Contact Charlie Frago at cfrago@tampabay.com or (727)893-8459. Follow@CharlieFrago.