St. Petersburg, Florida (CNN) -- Declaring that his only loyalty is to the people of his state, Florida Gov. Charlie Crist announced Thursday that he will run for the United States Senate as an independent rather than a Republican.

The move sets the stage for an unprecedented three-way race in which Crist will test whether he can win federal office without the formal backing of a political party.

"I could have chosen to stay in the [Republican] primary, but frankly for me, this is your decision," he told a crowd of about 300 supporters and campaign volunteers gathered in a picturesque St. Petersburg park overlooking Tampa Bay. "It's not one club's decision or another."

When it finally came, Crist's declaration of independence was somewhat of an anti-climax. With poll after poll suggesting he stood no chance of winning the GOP nomination over his ascendant rival, former House Speaker Marco Rubio, Crist began hinting at an independent bid weeks ago. News of his decision leaked a day early.

Crist told reporters after the speech that he made up his mind to run without party affiliation "late last night." Asked if he could raise the kind of money needed to run a competitive campaign in the nation's fourth largest state, Crist responded: "I already have."

Rubio: It won't change my strategy

He would not say whether he would caucus with Republicans or Democrats if elected, telling a reporter that he will "caucus with the people."

After he made it official on Thursday, reaction from the Republican establishment was swift and fierce.

"I am not surprised," said former Florida Gov. Jeb Bush, who is backing Rubio. "This decision is not about policy or principles. It is about what he believes is in his political self-interest."

The Republican Senate leadership said it will ask Crist to refund all of its campaign donations, and it called on the governor to return money to the "thousands of Republican donors" who have supported his campaign.

The leaders, including Senate Minority Leader Mitch McConnell and National Republican Senatorial Committee Chairman John Cornyn, promised to throw their full support to Rubio, whom they called "an emerging star."

"The question for Floridians is whether [Crist] will keep his word about all of the new promises he makes," the GOP Senate leaders said in a statement. "Elections are about trust, and frankly, it is unclear whether Gov. Crist deserves any."

Most of Crist's campaign staff, including his campaign manager, Eric Eikenberg, and campaign attorney, Benjamin Ginsberg, submitted their resignations shortly after Crist made his intentions public. Crist also lost his Republican polling firm, Public Opinion Strategies.

"It has been an honor to work for Gov. Crist, and I wish him all of the best," said Communications Director Andrea Saul, who resigned moments after the announcement.

Others came to Crist's defense, including the embattled former chairman of the Republican Party of Florida, Jim Greer, who was appointed to his post by Crist. Greer was forced to step down from the chairmanship earlier this year after questions arose about excessive spending at the party and now faces a criminal investigation into his stewardship of the Florida GOP.

"After facing political threats and intimidation by Republican leaders, Charlie Crist once again put the people of Florida first over partisan politics and mean-spirited, vindictive demands of the current Florida GOP leadership," Greer said.

Rubio, meanwhile, said the new reality of a three-man race will not change his election strategy, saying he'll continue to run "an ideas-based, issues-based campaign."

Speaking to reporters in Miami before his son's tee-ball game, he said the race will offer "a great opportunity to see our republic at work."

The Democratic candidate, Rep. Kendrick Meek, also promised that his message would remain the same. Meek argued that Crist has accomplished little as governor.

"The difference between the governor and I is that I have never been accused of being lazy," Meek told reporters in Washington. "I've always been out there working on behalf of the people of the state of Florida."

Crist, in his speech Thursday, signaled that he will run on populist themes and attempt to capitalize on some of the positions he has taken as governor in recent weeks.

"I don't have either party helping me, but I need you the people more than ever," Crist said at the rally.

The crowd in St. Petersburg included a number of teachers and clean energy advocates -- constituencies Crist will seek to tap into as an unaffiliated candidate.

The governor recently won favor with teachers unions in the state for vetoing a merit pay bill backed by conservatives, and earlier this week he reversed his support for offshore drilling after an inspection of the growing oil spill near the Gulf Coast.

"I haven't supported an idea because it is a Republican idea or a Democratic idea," he said at the rally. "I support ideas because they are good ideas for the people. I have always found that's what the people believe too."

CNN's John King, Martina Stewart and Shannon Travis contributed to this report.