TAMPA — Jeb Bush isn't the only prominent Florida politician facing a backlash from activists in his own party.

Democratic U.S. Rep. Patrick Murphy of Jupiter has emerged as the Democratic establishment's favored nominee for U.S. Senate in 2016 — including Tampa Mayor Bob Buckhorn, who endorsed him Tuesday — just as Bush is the GOP establishment's choice for the presidential nomination. And as Bush has conservatives criticizing him as too liberal, Murphy has liberals calling him too conservative to deserve the Democratic U.S. Senate nomination.

"When it comes to the race shaping up to replace Marco Rubio, we believe that Floridians are looking for a bold champion who will inspire and engage voters," said Susan Smith of Hillsborough County, president of a group called the Democratic Progressive Caucus of Florida. "We can't afford to lose more seats to Republicans by running former Republicans, or Democrats who otherwise can't be distinguished from Republicans."

Murphy, 31, is a former Republican who in 2012 unseated U.S. Rep. Allen West, a well-funded tea party favorite. He appeared at Curtis Hixon Waterfront Park on Tuesday alongside Buckhorn, another centrist Democrat, who said Murphy has proved he can win in tough races and raise the money necessary.

"Ideologically, he is exactly where most Floridians are," Buckhorn, 56, said of Murphy. "We are not a state of the extremes. We are a state of the middle."

Buckhorn had nothing to do with Charlie Crist's 2014 campaign for governor. As a likely candidate for governor in 2018, the Tampa mayor could use some chits with other leading Democrats.

Liberal activists complain that Florida Democratic Party leaders too often embrace centrists rather than more progressive candidates who might be more effective at energizing Democratic voters. Smith's group wants Democratic U.S. Rep. Alan Grayson of Orlando to run for Senate, but he remains undecided.

"I'm very grateful. I pay a lot of attention to what like-minded people want," Grayson told the Tampa Bay Times about the effort to draft him. "I understand and join in their sense that we need a true Democrat carrying our banner in 2016, someone who has a proven record of fighting for justice, equality, compassion and peace."

Murphy brushed off the skepticism about his Democratic values.

"I'm proud to support comprehensive immigration reform, I haven't changed my position on marriage equality, on a woman's right to choose, raising the minimum wage. These are all things that I support, along with getting our fiscal house in order," Murphy told the Times.

The second-term congressman was a registered Republican before running in 2012, and was asked Tuesday whether he had voted for George W. Bush for president.

"Do we have to get into that?" said a laughing Murphy, who contributed $2,300 to Mitt Romney in 2007.

Who was the first Democrat he voted for for president? "(John) Kerry would have been in there," Murphy said.

Rubio is expected to run for president, rather than seek re-election. Chief Financial Officer Jeff Atwater and U.S. Rep. Ron DeSantis of Ponte Vedra Beach are likely Republican candidates.

Times Washington Bureau Chief Alex Leary contributed to this report.