Sen. Marco Rubio (R-Fla.) came out swinging during the CNBC debate and had some big applause lines. See his most memorable moments of the third Republican debate. (Jhaan Elker/The Washington Post)

Donald Trump and Ben Carson are still dominating the polls, but many in the crowded Republican presidential race are now launching attacks on the candidate who seems poised to be their next major threat: Marco Rubio.

The freshman Florida senator has seen small but steady gains in the polls, attention from major donors and a growing collection of endorsements, especially after a well-received performance in last week’s GOP debate. That has quickly made him a prime target for criticism from his rivals, especially Trump and former Florida governor Jeb Bush.

During a news conference Tuesday in Manhattan, Trump called Rubio “overrated,” accused him of being “a disaster with his credit cards” and attacked him as “very weak” on immigration. At the same time, Bush — whose campaign has disparagingly labeled Rubio as a “GOP Obama” — doubled down on criticizing Rubio for missing more than a third of his Senate votes this year.

“People that are serving need to show up and work,” Bush said on CNN. “Period. Over and out.”

For months, Rubio has avoided direct confrontations with his rivals. His top aides have long worried about peaking too early, preferring to crest just before the first nominating contest on Feb. 1 in Iowa. The latest attacks bring a new level of attention and scrutiny to the 44-year-old senator’s below-the-radar campaign that could throw off his timeline.

At a news conference for the release of his book "Crippled America," Republican presidential candidate Donald Trump took aim at his 2016 opponents, saying it's time for some of them to drop out of the race. (Reuters)

“Well, you can only hide the bacon for so long,” said David Carney, a top strategist for past GOP presidential candidates who is neutral in this race. “They are fully exposed at this point. This is the gantlet that a serious campaign has to be prepared to run.”

Carney predicted the next hundred days will be “brutal” for Rubio. “If he withstands it, he will be a stronger candidate,” he said.

Two other GOP presidential candidates, Sens. Rand Paul (R-Ky.) and Ted Cruz (R-Texas), have also targeted Rubio in recent days — Cruz on trade and Paul on Senate votes. “If you’re going to accept pay, you ought to show up for work,” Paul told CNN.

The remarks signal that the Senate attendance criticism, in particular, may not be going away, even after Rubio successfully parried Bush on the issue at last week’s debate in Colorado. Trump said Tuesday that Bush’s attack during the event was “poorly delivered, but the facts are on Bush’s side.”

Rubio has said he has missed Senate votes both because he is running for president and because he is “frustrated” by congressional gridlock. He has also noted that many prior presidential candidates, including Sen. John McCain (R-Ariz.), also missed a lot of Senate work while campaigning.

Rubio campaign spokesman Alex Conant declined to respond to this week’s criticism.

“As Marco said at the debate last week, he’s not running to attack any of the other candidates,” Conant said Tuesday. “He’s running for president because he believes he best understands the challenges facing America in the 21st century. We feel very good about our campaign’s momentum, but obviously we have a lot of work to do and a long way to go.”

In the days since Colorado, Bush has continued to chip away at Rubio, though not as directly. During a speech in Tampa on Monday aimed at resetting his struggling campaign, Bush did not use the senator’s name but compared Rubio’s short Senate experience to that of President Obama.

“The challenges we face as a nation are too great to roll the dice on another presidential experiment,” Bush said.

And then there are Rubio’s personal finances, which have dogged him since he ran for the Senate in 2010. During that campaign, questions emerged about his personal use of a credit card issued to him by the Republican Party of Florida when he served in the state House. He re­imbursed the state party for flights he said were mistakenly billed to the party and paid for personal expenses that he had charged to the card.

Rubio reported in financial disclosures this year that he had cashed in retirement savings worth nearly $70,000 — a move commonly seen as a sign of financial distress. At the time, Rubio was paid $174,000 as a senator and also collected more than $50,000 from book royalties, a university teaching position and a rental property.

Although those amounts put his family among the top echelons of American earners, Rubio is one of the least wealthy members of the Senate. He has defended his financial history, saying that he is not from a rich family, had to take out loans to pay for his college education and now struggles to pay for his children to go to a private Christian school. He also said the retirement cash-out was due in part to having to spend $3,000 after his refrigerator broke.

“All you have to do is look at his credit card — I mean, he’s a disaster with his credit cards,” Trump said during the news conference Tuesday morning at Trump Tower to launch his new book, “Crippled America.”

“For years I’ve been hearing that his credit cards are a disaster,” Trump continued. “. . . He certainly lives above his means, there’s no question about that.”

The billionaire’s attack surprised Rush Limbaugh, who compared it to Democrats criticizing 2012 Republican nominee Mitt Romney for “not paying his taxes.”

“It is apparent that the Trump campaign believes that the establishment’s setting Rubio up to step into Jeb’s position if Jeb doesn’t make it,” the radio talk-show host said during his Tuesday broadcast.

If there was ever a time for Rubio to burst into the top tier, it’s now. He is enjoying the glow of three relatively strong debate performances and benefiting from the early collapse of candidates Scott Walker and Rick Perry and from Bush’s persistent struggles. Rubio won the backing of billionaire GOP benefactor Paul Singer last week and picked up the endorsements of Sens. Cory Gardner (R-Colo.) on Monday and Steve Daines (R-Mont.) on Tuesday.

Rubio has slowly risen in ­early-voting state polls in recent weeks, although he is still far behind Trump and Carson, neither of whom have ever held elected office. Still, Rubio is beating Bush, who is now mired in single digits in most surveys.

“I think so far all of the attacks have been very weak and ineffective. I’m not terribly worried at the moment,” said Eric Anton, a New York investment broker who is raising money for Rubio. “Let’s put it this way: Nothing new has come up.”

Ed O’Keefe and Katie Zezima in Washington contributed to this report.