Both major political parties are under pressure to find a Senate candidate to succeed Florida Republican Marco Rubio following his Monday announcement that he will not seek re-election.

Rubio, who made his 2016 presidential campaign official Monday, confirmed longstanding reports that he would leave his U.S. Senate seat to make way for his White House run. But neither Republicans nor Democrats are ready with a sure-fire candidate for the open Senate seat, and the Florida Senate contest could become one of the most expensive and closely watched races in the nation.

On the Republican side, uncertainty looms as to what kind of GOP nominee could best replace Rubio, who won his seat in 2010 as a Tea Party candidate.

Democrats, meanwhile, are bickering over whether to pick a moderate or liberal candidate as their own nominee.

The whole process is likely to cost millions of dollars, as Florida is considered a key presidential battleground state without which no candidate can win the White House.

President Obama won Florida in 2012 by less than one percent of the vote and many predict the state's 2016 races for the White House and Senate will be equally tight.

"It is definitely too close to call, and that means a lot of money is going to have to be poured into it," University of South Florida political science professor Susan MacManus told the Washington Examiner.

For Sunshine State Republicans, hopes were dashed last weekend when Florida's Chief Financial Officer Jeff Atwater announced he would not run for the seat.

Many in Florida's GOP considered Atwater to be the most recognized and popular Republican among an assortment of GOP names, but Atwater announced that despite encouragement from many people to jump into the race, he would "remain committed to only one endeavor and that is to be the best CFO I can be for the people of Florida."

Atwater's exit stirred a new round of interest, this time from Rep. Tom Rooney.

"My primary focus is on serving the people of Florida's 17th district, but I have received strong, positive feedback from many supporters and constituents about taking up the mantle of common-sense conservatism from Marco in the Senate, " Rooney said in a statement provided to the Washington Examiner. "Over the next several days and weeks, I will give careful consideration to how I can best serve our state and advance the causes that are critical to the future of our country."

Republican Reps. Vern Buchanan and Ron DeSantis have also expressed interest.

In addition to the three House lawmakers, Lt. Gov. Carlos Lopez-Cantera and former Florida House Speaker Will Weatherford could also seek the GOP nomination.

But even with a handful of possible candidates, the GOP faces a tough challenge after Atwater's withdrawal, state Republican strategist Alex Patton said.

Lopez-Cantera, for example, was first appointed to his seat and ran on the ticket under Gov. Rick Scott, which left him lacking in name recognition.

A Quinnipiac University poll released earlier this month found that 78 percent of Florida voters did not know enough abut Lopez-Cantera to have an opinion about him, compared to 58 percent for Atwater.

"Now that Atwater is out, you have four or five people on the Republican side with really no significant name identification," Patton said. I'd put the race in the solid toss-up category."

But Democrats aren't faring much better in their scramble to flip the seat back to their party.

While the nominees of both parties are to be decided in an Aug. 30, 2016 primary, the Florida Democratic Party has already coalesced around Rep. Patrick Murphy, a two-term moderate and former Republican who represents the state's 18th District in Southeast Florida. Murphy announced his candidacy in March.

The move has triggered Democratic infighting, with the party's progressive wing demanding a more liberal candidate, specifically Rep. Alan Grayson, who represents central Florida's 9th District.

Progressives expressed anger that the party appeared to be handpicking a candidate, rather than allowing the primary process to decide the nominee.

Last month, 63 percent of the state's progressive caucus picked Grayson to run as the Democratic nominee and hinted that Murphy is too moderate.

"This vote clearly demonstrates that progressive Floridians are looking for a bold champion who will inspire and engage voters," Susan Smith, president of the Democratic Progressive Caucus of Florida, said in a statement. "We can't afford to lose more seats to Republicans by running former Republicans, or Democrats who otherwise can't be distinguished from Republicans."

Grayson said he's considering running for the seat if another liberal Democrat doesn't jump into the race.

There are no distinct frontrunners so far in either party.

The Quinnipiac Poll found Murphy leading Lopez-Cantera by four points, while Lopez-Cantera beat Grayson by just one point.