Marco Rubio’s “lean” presidential campaign is putting on weight.

By every measure, the Florida senator’s bid for the Republican nomination has grown more robust in October, boosted again by a strong showing in Wednesday night’s debate. In preference polls and money flowing in, he’s ticking upward.

The campaign’s fundraising, which lagged that of several competitors over the summer and early fall, just finished its best month yet. In the hours around the debate Rubio raised $750,000 online — more than on any previous occasion.

The next day, Rubio was cheered at two fundraisers in Chicago, each of which had more attendees than organizers had planned. At a “young professionals” happy hour at a downtown sports bar, the 44-year-old Rubio told a rowdy group that he woke up that morning “still kind of wired” from the debate.

“It’s becoming easier to get people to say yes,” said Chris Grozev, who said he sold a couple hundred $100 tickets to the happy hour.

Phil Rosen, a New York real estate lawyer who hosted one of the most lucrative fundraisers for the campaign a few weeks ago, said he’s since had “people come out of the woodwork and call me directly, asking for another event so that they can meet him.”

And billionaire investor Paul Singer on Friday announced his support for Rubio in a letter to his extensive network of Republican fundraisers, encouraging them to follow his lead.

Donor enthusiasm has given campaign leaders who have prided themselves on a slim and sleek operation — partly out of necessity because of low cash flow — the confidence to increase hiring.

Rubio’s staff grew by about one-third in October, making for crowded conditions in the Washington row house that serves as headquarters. Rubio is steadily adding resources in each of the first four voting states, and the campaign just signed an office lease in South Carolina, the third state voting in the primaries early next year. Volunteers there had been working out of a garage.

“We’re definitely building,” said Terry Sullivan, Rubio’s campaign manager. “But we’re scaling at the right time.”

With growth comes the potential for growing pains.

Rubio’s competitors are sharpening their criticism of the freshman senator.

In Wednesday’s debate, mentor-turned-rival Jeb Bush went after him for missing Senate votes while he campaigned for president.

The former Florida governor came to the debate after briefing his top fundraisers on a strategy that hinges on overpowering Rubio, whom his campaign sees as his most dangerous competitor for the voters and donors who want to see a traditional nominee.

Bush’s attack backfired during the debate. But other contenders — from Democratic front-runner Hillary Rodham Clinton’s allies to Florida newspapers — have seized on that line of attack, and it is likely to persist.

Donald Trump, the celebrity real estate mogul who continues to lead in national GOP polls, also is increasingly scornful of Rubio. In an interview with Breitbart News published this week, Trump called Rubio “incapable of telling the truth.”

However, Rubio-rippers have their work cut out for them: He was one of the best-liked candidates in an Associated Press-Gfk poll conducted this month, with registered Republicans viewing him more favorably than unfavorably by a 31 point margin, second only to retired neurosurgeon Ben Carson.

And Rubio has said he’s not going to make his nomination fight personal. He said Thursday on ABC’s “Good Morning America” that he has “admiration” for Bush and wants only to underscore policy differences between the two.

Rubio has sketched out the basics of proposals on China, education and taxes, among other issues; Saturday is the last day of his “31 Days of Policy.” But he’ll have to go deeper into detail as the race intensifies.

Singer praised Rubio’s grasp of the issues and his status as “one of the best communicators the modern Republican Party has seen,” in a letter distributed Friday to his donor network, first reported by The New York Times.

“Marco Rubio can appeal to both the head and the heart. He can lead our nation by inspiring it,” wrote Singer, who is among the most influential donors in Republican politics.

Rubio said Friday during a campaign appearance in Iowa he was grateful for the billionaire’s backing. “It’ll help us with resources,” he said. “Resources alone are not enough. You have to have the right ideas and the right principles.”

Rubio has been slow to outline a path to his party’s nomination.

His campaign argues he could do well in any of the first four states, a quartet that also includes his childhood home of Nevada. Accordingly, he has divided his time roughly equally among them and has six or so employees in each. Bush’s campaign, in its presentation last week to donors at a Houston gathering, noted 12 staffers in New Hampshire and another 25 spread among the other three early states.

Rubio’s equal-attention approach to the early states also shows up in the television advertising plan.

Campaign ads are to begin the week after Thanksgiving, with plans to spend about $17 million through the early primary contests, according to information provided by Kantar Media’s CMAG advertising tracker. About the same amount of money is scheduled for Iowa, New Hampshire and South Carolina.

A super PAC filled with Rubio allies follows a similar advertising strategy, CMAG shows, with about $2 million to $3 million in ad time reserved in each of the first three states.

The campaign and super PAC are stepping up after a summer of silence.

To date, the only Rubio-boosting commercials on television have come from a nonprofit group that keeps its donors secret. Conservative Solutions Project had spent $7.3 million through the end of this week.

No other presidential candidate has benefited from so much advertising by a secret-money nonprofit, a potential liability with voters who have said in polls that they are tired of big donors and secret money flowing into elections.

(AP)