Marco Rubio, Ted Cruz ramp up attacks on each other in bid to dethrone Trump

It was a foreign-policy shot at Marco Rubio's views on Syria.

"The enemy of my enemy is not necessarily my friend," Texas Senator Ted Cruz tut-tutted, criticizing Rubio for supporting the arming of rebels against Syrian President Bashar al-Assad.

But the remark just as easily could have defined the increasingly fraught relationship between the two Cuban-American senators vying to dethrone Donald Trump.

With the attention-grabbing real estate tycoon leading the race for the Republican presidential nomination, Rubio and Cruz both of whom describe each other as "friends" are tangling over second place, each positioning himself as the viable alternative should Trump falter.

But as the pair redirect their attacks towards each other, observers warn they could both end up losers.

"It's like right now they're trying to murder each other," says Washington Democratic strategist Joe Trippi. "If they both end up dead, it becomes a suicide pact."

For now, in the lead-up to the last Republican debate of the year tomorrow, Rubio and Cruz appear to be consolidating support.

Rubio, the Miami senator who won the backing of influential billionaire Paul Singer, is emerging as something of an establishment darling.

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Cruz, a Tea Party favourite who has embraced his reputation as "the most hated man in the Senate," boasts outsider appeal designed for the conservative Christian set, an important force in the upcoming Iowa caucuses.

Republican and Democratic pundits project that if or when a Trump candidacy collapses, the Texan and the Floridian will be the first and second most likely candidates to win the nomination.

Meanwhile, support for retired neurosurgeon Ben Carson, who was at one point neck-and-neck with Trump, appears to be slumping.

The fight for second spot

A recent Quinnipiac University poll put Trump at 27 per cent support among likely GOP primary voters nationwide, followed by Rubio in second place at 17 per cent, then Cruz tied with Carson for 16 per cent.

Last week's Monmouth University poll had Cruz vaulting to the front of the pack with 24 per cent to Trump's 19 per cent. Rubio was trailing slightly at 17 per cent in December, but jumped seven points from October.

Cue the ramped-up hostilities.

"I stand strongly on behalf of the ability of this government to gather intelligence on our adversaries, our enemies, especially terrorists," Rubio told Fox News following the Paris attacks.

"There are Republicans, including Senator Cruz, that have voted to weaken those programs," he went on. "That's just a part of the record; it's nothing personal."

Cruz struck back, calling Rubio's campaign "desperate" over a series of TV ads produced by the SuperPAC American Encore.

In the ads, the pro-Rubio organization contends "Cruz voted to weaken America's ability to identify and hunt down terrorists," a reference to Cruz's co-sponsorship of the USA Freedom Act, which set limits on domestic surveillance.

"Senator Rubio's campaign is very, very dismayed at conservatives coming together behind our campaign," Cruz responded, adding that Rubio was "attempting to mislead voters in an effort to slow that [momentum] down."

In another broadside, Cruz speculated that Rubio wants to divert focus away from what he characterized as Rubio's liberal-leaning record on immigration reform.

"Senator Rubio's campaign has been desperate to change the topic from his longtime partnership and collaboration with President Obama and [Democratic Senator] Chuck Schumer in pushing a massive amnesty plan," Cruz said.

Helping Trump?

From Washington, Trippi has been watching the freshman senators exchange blows with particular interest.

Whenever any two political opponents engage "in a big knock-down, drag-on dogfight," he said, "it's rare that either one of the two benefits; it almost always accrues to somebody else."

And in this case? "It could definitely be Trump" who stands to gain the most, he says.

Trippi would know. It was precisely that scenario that played out in the 2004 Democratic Party primaries, when negative campaigning between perceived frontrunners Howard Dean and Dick Gephardt eroded enough support from either side that John Kerry and John Edwards ascended instead.

You want to be an outsider in this race; you don't want to be seen as the insider. Democratic campaign strategist Joe Trippi

Trippi was Dean's campaign manager at the time and learned a lesson then.

"It's better to try to go negative against the entire field, as Trump does," he said.

"Picking one guy and going negative, then that guy responding and the two getting in a huge fight, that gets dangerous."

Trump's former top campaign aide, Roger Stone, also believes Trump may become an "indirect beneficiary" of the Cruz-Rubio clashes.

By the same token, Trump's attacks on Carson may have only bolstered Cruz's candidacy. "As votes fall off Carson, they're moving to Cruz, not Trump," he said.

The trio Trump, Carson and Cruz are splitting the conservative "outsider" vote, pundits are saying.

For that reason, Stone who says he quit the Trump camp in August over a disagreement about the handling of the candidate's row with Fox News host Megyn Kelly believes it is still Trump who presents the greatest obstacle for Cruz.

Rubio is in the trickier situation, he suggests. Even if Rubio succeeds in harming Cruz, that far-right faction is more likely to wander to Trump or Carson.

"Marco's numbers in Iowa and New Hampshire are improving, but the fact he's been picked up [for endorsement] byPaul Singer kind of shows he's the boy-toy of the billionaires and the next establishment candidate," Stone said.

From the other side of the political aisle, Trippi agrees: "You want to be an outsider in this race; you don't want to be seen as the insider."

Philip Ammerman, a political strategist and vice-president of Navigator Consulting, attended Princeton University with Cruz. Both were members of the Whig Clio debating society, though Ammerman doesn't recall Cruz having any strikingly conservative inclinations at the time.

Nor does he recall much about Cruz being "unlikable."

"Ted appeals naturally to religious conservatives," he says. "He was a very reasonable man, very friendly in person, and still is. He's certainly not the ogre he's made out to be now."

Ammerman also notes that even if the Cruz and Rubio campaigns declare "a ceasefire" between them, their PACs will likely keep sniping from the sidelines.

"Trump has set the standard for negative rhetoric, which means everybody will be playing to that, and will be continuing it," he says. "I just don't see where it ends."