Jeb Bush's super PAC turned up the heat on Marco Rubio on Monday, unveiling a new $3 million-plus television spot to the Washington Examiner that accuses the Florida senator of being an immigration flip-flopper who supports amnesty.

The advertisement might raise eyebrows since Right to Rise USA is hitting Rubio on an issue on which he and Bush, both rivals for the Republican presidential nomination, agree. Rubio and Bush, the former Florida governor, have both expressed support for a pathway to citizenship for some illegal immigrants. The position has proven to be a political vulnerability for both candidates, but more so for Bush, who trails Rubio in state and national polls.

Strategists for Right to Rise USA view the spot as a critique of Rubio's character as a leader, rather than about immigration. The message the super PAC hopes to sell is that Rubio can't be trusted to stand his ground on in the face of opposition or during tough times. By extension, Bush's position might be unpopular, but he's consistent and has the backbone voters want in a commander-in-chief.

Bush isn't competing heavily in Iowa, where this ad is targeted. That makes this an obvious play to squash Rubio's candidacy before the New Hampshire primary, which Bush is depending on to revive his sagging presidential campaign. Iowa caucuses on Feb. 1; New Hampshire votes on Feb. 9.

"Marco Rubio; he ran for Senate saying he opposed amnesty. Then he flipped, and worked with liberal [New York Sen.] Chuck Schumer to co-author the path to citizenship bill. He threatened to vote against it and then voted for it," the ad's voiceover says, as a smiling picture of Rubio swings back and forth on a weather vane.

"Marco Rubio, just another Washington politician you can't trust," the voiceover says as the spot closes. "Jeb Bush, he's a leader, so you always know where he stands."

The Rubio campaign fired back, calling the advertisement disingenuous. "Jeb's PAC 'Right to Rise' should rename itself 'right to lie.' The truth is that Bush, Cruz and Christie have all flip-flopped on immigration reform," Rubio spokesman Alex Conant said, lumping Bush, Sen. Ted Cruz of Texas and New Jersey Gov. Chris Christie together.

The television ad is going on television immediately in Iowa and South Carolina, and nationally on Fox News Channel. In Iowa, Right to Rise USA is investing $1.8 million for a two-week run on broadcast and cable. In South Carolina, the super PAC is spending more than $1 million for one week on broadcast and cable. The spot will eventually hit the airwaves in New Hampshire, where Bush is spending most of his time campaigning as he seeks a come-from-behind win to vault back into contention for the nomination.

The spot, to run on radio, digitally and via direct mail, risks angering some Bush donors who also like Rubio and didn't necessarily expect their investment in his super PAC to be used to tear down Rubio — at least absent a head-to-head matchup for the nomination. These Republicans view Rubio as the future of the GOP, and a strong alternative to Bush, and worry that Right to Rise USA's attacks on the senator will simply clear a path for Cruz, who is running second in most state and national polls, or front-runner Donald Trump, the celebrity businessman from New York.

Right to Rise USA isn't concerned. Strategists say contributors to the super PAC have been pleased with previous attack ads it has run against Rubio, and they don't expect any blowback for this latest spot, even though it attempts to undermine the senator for backing a policy many of the group's donors probably support using language similar to that used by Cruz and other conservatives who oppose the legalization of illegal immigrants.

Bush is a longtime proponent of comprehensive immigration reform that would provide a path to citizenship to some illegal immigrants. In 2013, Rubio adopted that position as a part of his participation in the bipartisan, "gang of eight" negotiations that led to the passage of a comprehensive immigration reform bill in the Senate. It later died in the House. Rubio still backs that policy, but now supports securing the border first as a part of a step-by-step legislative approach to overhauling U.S. immigration law.

Bush previously praised the "gang of eight" legislation.

After tens of millions of dollars in spending by Right to Rise USA and the Bush campaign, Bush is stalled in the middle of the crowded GOP field and rated among the most unfavorably of any of them. He's redirected most of his campaign resources to New Hampshire, so far to minimal results. Rubio is running second in New Hampshire, and third in Iowa and South Carolina, and continues to pursue a strategy of competing in all four early primary states, including Nevada.

Strategists for Right to Rise USA remain confident that Bush can turn things around by the time primaries get under way next month.

Although multiple candidates stand in Bush's way in New Hampshire, including New Jersey Gov. Chris Christie and Ohio Gov. John Kasich, not to mention Cruz, Rubio and Trump, the super PAC's brain trust views Rubio as the governor's main obstacle. Clear him out, and a strong finish both in the Granite State and in South Carolina, where the Bush family has deep ties, becomes possible. In a head-to-head matchup with Cruz or Trump, or even a three-way contest with those two, Right to Rise USA likes Bush' chances.

These strategists concede that Bush's personal favorable numbers appear problematic. But they blame that dynamic on the fact that most Trump supporters dislike Bush, while expecting the challenge to be overcome by the passionate support of the voters who do like him.

"In a race with this many people, everyone has a different principal opponent. Bush — and Christie — have to beat Rubio. Rubio has to beat Cruz. Cruz has to beat Trump. And Trump has to beat 50," said a Republican operative who is neutral in the primary.