An image from a commercial in which the super PAC Right to Rise, which supports former Florida governor Jeb Bush, compares the positions of Sen. Marco Rubio (R-Fla.) to a weather vane rotating with the wind.

The super PAC supporting former Florida governor Jeb Bush plans to keep up its advertising assault on onetime protege Marco Rubio and other establishment-friendly GOP presidential candidates, ignoring calls from many Republicans to focus on Donald Trump instead, according to people familiar with its strategy.

The outcome of efforts by Right to Rise — which raised a record $103 million in the first half of 2015 — will serve as a key test of whether deep-pocketed super PACs can alter the dynamics of a presidential race.

The group’s spending — $64 million so far on ads and mailers — has provoked a sharp backlash among allies of the senator from Florida and some top GOP donors, who say it should be aiming its substantial firepower at front-runner Trump or the Democratic candidates.

The super PAC still plans to eventually shift its focus to Trump and Sen. Ted Cruz of Texas, who are leading in the polls heading into Monday’s Iowa caucuses. But the pair will not become the group’s main targets until it can help Bush clear the field of other rivals, officials said.

“We are the only organization that has spent a considerable sum against Donald Trump, and Jeb Bush is the only candidate who has had the guts to take him on,” PAC spokesman Paul Lindsay said in a statement. “But this is still a jump ball among the other candidates in the field, and we think it’s important that Republican primary voters have information on all of their records before making their decision.”

Former Florida governor Jeb Bush in Des Moines. (Charlie Neibergall/AP)

The group’s strategy can be seen in a new ad debuting in Iowa this week, which seeks to frame the race as a competition over who is best to take on the two poll leaders.

“Can’t stomach Trump or Cruz? Look at three governors,” an announcer says at the start of a spot, which is scheduled to begin airing in some parts of the state Thursday. The commercial goes on to attack the records of New Jersey Gov. Chris Christie and Ohio Gov. John Kasich, who like Bush and Rubio are vying for the votes of mainstream Republicans.

“Jeb Bush: a proven conservative you can trust,” the ad concludes.

It remains to be seen whether Right to Rise’s muscle will be able to boost Bush. In Iowa, where the group has spent at least $14 million to place ads on TV, radio, websites and billboards, Bush remains mired near the bottom of the large GOP field. The latest surveys of likely caucus participants show Rubio with roughly 13 percent support, unchanged from last month and placing him third, behind Trump and Cruz. Bush stands in fifth place with about 4 percent, down several points from last fall.

Nationally, the super PAC claims that it has spent about $5 million so far attacking Trump in television and digital ads, a figure that could not be verified in Federal Election Commission reports. But Right to Rise recently has devoted much of its arsenal to attacking Rubio, whom many Republican donors and activists think is best positioned to defeat Democratic front-runner Hillary Clinton.

The barrage against Rubio began last year and has focused mostly on his frequent absences from the Senate. A recent spot featured a cartoon of Rubio twisting on a weather vane as an announcer described his shifting position on immigration reform.

The super PAC began airing an ad in Iowa this week that focuses on Rubio’s use of a Florida GOP charge card he held between 2005 and 2008 to pay for personal expenses. Rubio later repaid the charges, totaling more than $22,000, and he publicly disclosed the details late last year. The ad charges that because of the spending, “He’s just not ready to be president.”

Republican presidential candidate Sen. Marco Rubio of Florida in West Des Moines. (Aaron P. Bernstein/Reuters)

The attacks have caught the attention of some Republican voters here. At a Tuesday town hall in Oskaloosa, the first question that Rubio faced was about the Right to Rise weather-vane ad.

“A PAC is attacking your involvement in ‘Gang of Eight.’ Please clarify that,” asked Madonna Bowie of Oskaloosa, referring to the bipartisan Senate group, which included Rubio, that pushed for an immigration bill.

Rubio responded that, as ­president, he would not pursue the comprehensive approach he backed in 2013. The attack ads, he said, are designed to “get people riled up about something we’ll fix when I’m president.”

In an interview after the event, Bowie said that she plans to support Rubio but she was hoping for a different answer that would assuage her concerns.

“I wanted him to kind of go into depth about the whole, the ‘Gang of Eight,’ what the bill was and explaining why he said what he did later. Just to clarify for people,” Bowie said.

On Wednesday, Rubio said the attacks are “almost free advertising for Hillary Clinton.”

“If you add up all the money that’s been spent against every other candidate, it does not equal to what’s been spent against me — over — close to $30 million now of attack ads, mostly from Jeb Bush,” Rubio told an Iowa radio show. “No one else has faced anything close to that, not Donald Trump or Ted Cruz or anyone else.”

Mike Murphy, a longtime adviser to Bush who serves as the chief strategist of Right to Rise, declined to comment.

Exactly how much the group has spent against Rubio cannot be determined precisely from its FEC filings. In all, Right to Rise has reported spending $64 million so far on ads and mailers to bolster Bush and go after his opponents, including at least $15.3 million on spots that swipe at Rubio and other Republican candidates. But the reports do not reflect new ads that may have been swapped on the air for other spots initially reported.

As of Tuesday, the super PAC has spent at least $14 million in Iowa, $26.5 million in New Hampshire and $9.5 million in South Carolina.

While there has been ambivalence among Right to Rise donors about the group’s aggressive hits against Rubio, some of the group’s major backers said that they remain confident in its strategy.

“In campaigns, there is always a lot of second-guessing: ‘They should have done this, they should have done that,’ ” said Florida-based investor Howard Leach, a former ambassador to France who gave Right to Rise $100,000. “My view is that the professionals running the PAC know what they’re doing.”

Although Leach said the attacks against Rubio are “not attractive,” he chalked them up to the typical rough-and-tumble nature of a campaign.

“My advice to Senator Rubio was, ‘If you want to be president of the United States, you should be governor of Florida,” he said. “I like and admire Senator Rubio, but I think he’s inexperienced.”

Rubio supporters, though, are bristling at the barrage coming at him from the pro-Bush super PAC.

“I don’t think it has any impact, but I do think it’s counterproductive,” said Marc Goldman, a Boca Raton, Fla.-based investor supporting Rubio’s campaign, who said the firepower should be aimed at the Democrats. A better approach, he said, would be “clarifying what the problems are and how you would be the best one to turn it around, rather than knock somebody. I just think it’s not helpful.”

Bush, who helped Right to Rise raise money before he became an official candidate, has focused relentlessly on attacking Trump on the stump, with little success. But voters have taken notice of the super PAC’s focus on Rubio. Campaigning a few weeks ago in Ankeny, a woman asked him to explain his relationship with the senator.

“I like Marco. We live almost in the same Zip code, and I admire him: He’s a great politician, a great speaker. He’s a great talent,” Bush said. “But has he ever had a chance to make a tough decision? I don’t know. Think about it. “

Gold reported from Washington. Sean Sullivan in Oskaloosa, Iowa, and Anu Narayanswamy and Scott Clement in Washington contributed to this report.