Washington (CNN) Top Democrats are plotting a new effort to derail Marco Rubio if he seeks re-election to his Senate seat, saying they will pour millions to defeat the Florida senator in the hopes that a defeat would end his once-rising political career.

With Rubio now reconsidering a run for the Senate, Democratic leaders have sharpened their focus on Florida, piecing together an anti-Rubio campaign that attacks the Florida senator for skipping hearings and voting against a high-profile gun bill -- and tying him to presumptive GOP nominee Donald Trump. But above all else, Democrats say, their donors and supporters will be motivated by the expectation that a Rubio loss would end the senator's chances of ever becoming president.

"If Marco decides to run, it's going to be tough," Montana Sen. Jon Tester, head of the Democratic Senatorial Campaign Committee, told CNN Tuesday. "If he gets beat twice in one year, it's not good for him," he added, a reference to Rubio's defeat in his home state during the Republican presidential primary fight.

"Democrats are going to really go after him," said one high-ranking Democratic source who asked not to be named discussing internal strategy. "Because if he loses this time, he's gone for president in 2020."

Democrats, of course, have a reason to issue their warning now: They have a better chance at winning back the Senate seat if Rubio decides to retire at the end of the year. But the DSCC and other Democratic outside groups, including American Bridge and the Senate Majority PAC, are preparing to spend tens of millions in the expensive state against Rubio, and officials say the attacks against the senator in the perennial swing state will be relentless.

The Democratic National Committee, which dug heavily into Rubio's record in the presidential campaign, has already been in contact with the DSCC to share opposition research, according to a source familiar with the matter.

John Morgan, an attorney and major Florida Democratic donor, said that Rubio will be "done forever" if he loses a bid for a second term. To make that happen, he said, donors would pour big bucks behind Rep. Patrick Murphy, the favorite of the Democratic establishment, who is facing progressive firebrand Rep. Alan Grayson in the August primary.

"I think if Marco Rubio runs, Patrick Murphy will have more money than he can spend in Florida," Morgan said.

Rubio has until June 24 to make a decision on whether to run, but the calculation is complicated. Rubio confidantes say he'd like to run for president again, potentially in 2020 if Trump loses this fall, and they believe he could be better positioned to do so by not having the baggage of holding a political office. Yet if the GOP loses his seat, and the Senate majority, Rubio could be accused of not doing more to help his party stay in power. Plus, he could lose a national platform that being a senator provides.

On Tuesday, Rubio and his aides declined to comment.

"We're not talking about campaigns today," Rubio told CNN Tuesday.

But increasingly, Rubio has kept the door open to running. After insisting repeatedly that he would be a "private citizen" in January if he lost his bid at the White House, the Florida senator has come under intensifying pressure from Senate Republicans and top donors to change his mind.

Senate Majority Leader Mitch McConnell and other top Republicans fear that the current crop of Florida Senate GOP candidates are underfunded and have performed poorly thus far, arguing that only Rubio stands a chance of keeping the Senate seat from flipping to the Democrats.

"I hope he runs," said Mississippi Sen. Roger Wicker, chairman of the National Republican Senatorial Committee. "I think he's struggling with the decision right now. I think if he runs, he'll be a heavy favorite."

Adding to the pressure, GOP outside groups are planning to spend heavily to help Rubio, but some warn they'd sit out in the general election if he decides not to run.

"There is literally one person in America who has the ability to dramatically increase the chances of Republicans keeping the majority -- Marco Rubio," said Steven Law, a former chief of staff to McConnell who now runs the super PAC Senate Leadership Fund. "If Marco's in for Florida, we're in for him -- money, marbles and the chalk."

Rubio friends say it's uncertain what the senator will ultimately do. But after returning to the Senate following his loss in the Florida primary to Trump, Rubio has become more engaged in his job, taking a leading role pushing for funding to combat the Zika virus and now helping in the federal response to the massacre at an Orlando nightclub last weekend.

Speaking to conservative radio host Hugh Hewitt Monday, Rubio suggested that the fallout from the deadly Orlando shootings had made him rethink his own life choices.

"I haven't even given it thought in that perspective, other than to say I've been deeply impacted by it and I think when it visits your home states, when it impacts a community you know well, it really gives you pause, to think a little bit about your service to your country and where you can be most useful to your country," Rubio told Hewitt.

"You know, honestly, I haven't thought about it from a political perspective but it most certainly has impacted my thinking in general about a lot of things," Rubio added before saying: "My family and I will be praying about all this. We'll see what I need to do next with my life in regards to how I can best serve."

Rubio has cited his longtime friendship of a GOP candidate in the Senate race, Lt. Gov. Carlos Lopez-Cantera, as a prime reason why he wasn't planning on running. But top Republicans want the lieutenant governor to step aside and make way for Rubio, and many believe the candidate would drop out if the senator asked his close friend to make way for his re-election bid.

Courtney Alexander, a spokeswoman for the lieutenant governor, did not respond to multiple inquiries on whether her boss would definitively stay in the race.

Only Rep. David Jolly has signaled he'd drop his Senate run if Rubio jumped in, but the other GOP candidates in the race have insisted they'd stay in no matter what Rubio does.

"We're not concerned with Washington insider chatter," said Brad Herold, campaign manager for GOP Rep. Ron DeSantis, a Senate candidate.

So if Rubio runs, he's bound to face criticism on both the right and the left.

Democrats plan to hammer Rubio over his opposition to a bill that would deny terrorist suspects of purchasing firearms. DSCC spokeswoman Sadie Weiner said Rubio has an "impossible record" to defend, citing his missed votes, support for Trump -- whom Rubio himself once called a "con artist" who should not have access to the country's nuclear codes -- and his opposition to the gun bill. An official with American Bridge said that the group will also push on traditional Democratic attacks over Social Security and Medicare, as well as social issues such as same-sex marriage. And the high-spending Senate Majority PAC has already reserved more than $10 million in television airtime in Florida, adding to the $10 million booked by the DSCC.

On Monday, Senate Minority Leader Harry Reid took to the floor and lashed out at Rubio, an unusual rebuke but a sign of things to come if the GOP senator runs for reelection.

"How can the junior senator from Florida -- who all of a sudden is again interested in running for re-election -- how can he speak of running for office again when he voted to let potential terrorists buy assault weapons and explosives?" Reid said.

Republicans oppose the gun bill because they argue it will sweep up innocent people unfairly, prompting them to lose their constitutional rights. And Rubio's GOP allies said that Democrats are attacking the senator now because they are scared of his potential candidacy.

"Most everyone here would want to see him run for reelection," said Sen. Jim Risch, R-Idaho.