Clinton World is stepping up its attacks on Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE, signaling the Democratic presidential front-runner sees the rising Florida Republican as a significant threat.

Priorities USA, a super-PAC aligned with Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE, has issued a dozen separate email blasts or advertisements attacking Rubio by name in the last two weeks. No other GOP presidential candidate is receiving that kind of scrutiny.

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The group has taken swipes at Rubio’s use of a Republican Party credit card and his record in the Florida Legislature. Headlines have included “What Is Marco Rubio Hiding?”, “Rubio Tax Plan Debate Claims ‘Nonsense’ ” and “Rubio’s evasiveness and apparent misstatements.”

It’s a shift for the super-PAC, which in previous months seemed to be focused more on Rubio’s onetime mentor, former Florida Gov. Jeb Bush.

Bush has fallen in polls, however, while Rubio is rising.

The telegenic Cuban-American is a new threat to Clinton, and some believe he would be the GOP’s strongest candidate against the veteran Democrat in the general election.

A Democratic strategist close to the campaign said the attacks suggest that Clinton and her allies see Rubio as likely to win the nomination, given Bush’s problems, and that they believe the party will pick an establishment figure over an “outsider” candidate such as businessman Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE, retired neurosurgeon Ben Carson or Texas Sen. Ted Cruz Rafael (Ted) Edward CruzTrump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE.

“It’s really a look at who’s realistic,” the strategist said.

Rubio is surging after strong performances in the primary debates, including one where he scrapped with Bush about his voting record.

He’s picked up several endorsements from GOP senators and has gained ground in national polls, though he remains behind Trump and Carson by double digits in several surveys.

Clinton once seemed to see Bush as her most likely general election rival, training most of her fire on him.

But lately, attacks on Bush have grown less frequent.

She rarely mentions Trump or Carson by name, unless she is asked a direct question, though she did refer to Carson in a text to supporters during the most recent Republican debate in Boulder, Colo.

Priorities USA was once obsessed with candidates Bush, Wisconsin Gov. Scott Walker and Trump, but it is now focused on Rubio.

One of the three attack ads featured on the home page of the super-PAC’s YouTube channel is titled “Marco Rubio, Mansplainer” and stars only Rubio.

No other Republican is the sole focus of an entire ad on the Priorities channel. The other two attack ads on the main page describe how several of the Republican candidates — including Rubio, Bush and Carson — are “wrong” for senior citizens and for ending gun violence.

Rubio has only recently taken a prominent place in the super-PAC’s attacks.

A Priorities USA advertisement titled “This is the Republican Party” posted on YouTube two months ago did not mention him, instead focusing on Bush, Walker and Trump.

But Rubio increasingly appears to be in his own category.

“It has to do with Jeb Bush’s sinking poll numbers more than anything,” said Eric Jotkoff, a Democratic strategist and former Florida Democratic Party staffer who served as an aide on the 2008 Clinton campaign.

When Priorities USA released a press statement following the recent Republican presidential debate hosted by CNBC, the pro-Clinton super-PAC mentioned Rubio by name twice. The only other GOP candidate mentioned was Bush, and he was noted almost as an afterthought.

The Clinton campaign’s Rubio obsession appears to have caught on at the Democratic National Committee and with other influential allies.

On Monday, EMILY’s List, an influential group that backs female candidates and has endorsed Clinton, sent an email describing Rubio as the “new Scott Walker.”

“We already knew that all Marco Rubio’s policy agenda consists of is pursuing failed, conservative policies that hurt women and families,” Marcy Stech, communications director for the group, wrote in an email to reporters. “Now he’s apparently branched out to pursuing endorsements from failed, conservative candidates as well.”

Earlier in the day, the Democratic National Committee (DNC) sent three separate emails targeting Rubio.

A DNC official acknowledged that there has been a “focus on where the race is focusing” but “that doesn’t mean we’re not going to hold others accountable and focus on other candidates still in contention.”

“We’re by no means predicting what’s going to happen,” the official said, adding that Trump and Carson have held on to the top two spots for months.

Still, the official said, Rubio is “obviously worth looking at.”