TAMPA — Just when Republican presidential candidate Marco Rubio needs them the most, big-dollar contributors from the party’s wealthy mainstream are having second thoughts about his future in the 2016 race.

Fresh misgivings about Rubio’s path forward are the latest — and potentially the most debilitating — in a series of obstacles that threatens the Florida senator’s future in this rollercoaster Republican campaign.

“Super Tuesday came and Rubio didn’t do as well as some of us hoped. So people are saying, ‘Let’s see how this thing shakes out,’ ’’ said Craig Duchossois, who contributed $500,000 last year to a group that backed former Florida governor Jeb Bush.

“I’m holding back,’’ the Chicago-based investor said of his own plans.

Despite flashes of potential in recent weeks, Rubio has struggled to reconnect with the Tea Party movement voters who made him a favorite during their national breakthrough six years ago, instead watching them flock to presidential rivals Donald Trump and Ted Cruz.

Rubio campaign officials concede that Rubio probably cannot remain in the race without winning in Florida, where public polls show him second to Trump. Rubio’s team says the campaign’s polling shows the race tightening, with Trump leading by single digits, slightly less than recent public polling.

Rubio, elected to the Senate in 2010, also has not fully harnessed the financial muscle of the GOP old guard eager to derail Trump, despite the shift in focus by many to Rubio after Bush quit the race last month.

The result is a catch-22 for Rubio, who needs the money to win the March 15 primary in his home state of Florida, while donors wait out those results for signs of his long-term viability.

“We’ll see what happens on next Tuesday in Florida,’’ said Ron Gidwitz, another Chicago GOP donor who turned from Bush to Rubio. “We’ll see how real he is at that point.’’

In the meantime, the Trump campaign is trying to show that he can be a strong candidate in the general election. Should he win the Republican nomination, analysts say, he would be seeking support from working class, largely white voters in states that have long been Democratic bastions in presidential contests, from Maine to Pennsylvania to Michigan.

To make that work, he would have to not only hold on to those core supporters but also soften rhetoric that has alienated black and Latino voters and find ways to calm mainstream Republicans who say they will never vote for him.

Rubio had about $5 million in available cash at the beginning of last month, less than half of what Cruz had on hand. Trump has said he can afford to finance his own campaign, though he has received contributions.

Duchossois and Gidwitz were among a wave of mainstream GOP donors who moved quickly to look at Rubio when Bush quit the race Feb. 20 after failing to meet expectations in Iowa, New Hampshire, and South Carolina.

Rubio, on the other hand, finished a strong third in Iowa and rebounded from a disappointing fifth-place showing in New Hampshire to grab second place in South Carolina, feeding the GOP establishment’s hopes.

And yet Rubio’s momentum stalled again in the days leading up to March 1, when 11 states held Republican nominating contests. Afterward, Rubio turned from only indirectly critiquing Trump for months to an all-out assault on the businessman’s character and ethics, as well as his appearance and manliness.

Duchossois and others who pinned their hopes to him said they were turned off by Rubio’s taunts, including calling Trump’s “the worst spray tan in America’’ and equating Trump’s disproportionately small hands with his manhood.

“You just don’t do that,’’ said Bill Kunkler, another Chicago Republican who backed Bush but stopped short of the pivot to Rubio. “In Rubio, I don’t see the presidential gravitas.’’

Some potential Rubio donors also are concerned that Rubio can’t generate sufficient momentum for Florida based on his victories so far: Minnesota’s March 1 caucuses and the Puerto Rico primary Sunday.