By Jonathan Martin

New York Times

Some Christmas trees may still remain up, but the already contentious Republican presidential campaign is on the verge of entering a new phase, decidedly lacking in the holiday spirit.

The series of skirmishes that flared last week offered a preview of the more focused and intensive assaults that will come with the new year, as a volatile race featuring 12 candidates and divergent fronts in Iowa and New Hampshire turns even more combative and complex.

Supporters of Sen. Marco Rubio, Sen. Ted Cruz, Jeb Bush, Rick Santorum and Mike Huckabee are poised to unleash a wave of ferocious attacks this month, according to Republicans familiar with their planning, plunging the muddled contest into a multidimensional war in the weeks leading up to the Feb. 1 Iowa caucuses.

“You’re going to see even sharper elbows” starting this week, said Jon Seaton, a Republican strategist not affiliated with any of the campaigns. “And by the middle of January, everybody will have their pads on and helmets buckled.”

By month’s end, the candidates and their allied groups could spend as much as $100 million combined, much of it on negative advertising delivered via television, radio, mailers and digital spots. And after the effectiveness of positive television commercials came under question last year when tens of millions of dollars failed to lift Bush, January will offer insights into whether attack ads still have the power they did in past presidential elections.

Bush’s supporters effectively ended the Christmas truce last week when they went on the air in Iowa with an ad savaging Rubio over his Senate attendance record and with a New Hampshire commercial contrasting Bush’s achievements as Florida’s governor with Gov. Chris Christie’s tenure in New Jersey and Gov. John R. Kasich’s record in Ohio. The coming offensive will be just as varied, reflecting the layers of the Republican race.

Cruz is trying to head off Rubio, perhaps the only candidate who can build a coalition of conservative and center-right Republicans. Rubio is determined to slow Cruz in Iowa, where Cruz is at or near the top of every poll and could be formidable with a decisive win. But Rubio also must confront Christie, who is ascendant in New Hampshire, which votes about a week after Iowa.

And Christie and Bush are both focused primarily on denying Rubio the sort of strong showing in the two early states that could vault him toward a coveted position in what could be a three-way race: the mainstream Republican best positioned to stop Cruz or Donald Trump from winning the nomination.

The lines of attack put forward by the candidates and the super PACs supporting them, both on the campaign trail and through paid advertising, will showcase these strategic imperatives. But in such a fluid race, the targets could quickly change depending on who is rising or falling after this month’s two Republican debates.

And with so much uncertainty looming over the race — one poll last month indicated that fewer than 20 percent of New Hampshire Republican primary voters had definitively decided — the volleys and counterpunches could also propel a candidates who stays out of the fray.

Such an outcome — Candidate A’s targeting Candidate B only to help Candidate C — is always a possibility in a multicandidate field. But the conditions are especially ripe in a race where some of the party’s traditional dividing lines have been blurred, where there have been few negative ads until now and where the ability of the ostensible front-runner, Trump, to drive voters to the polls is in question.

“The law of unintended consequences is pretty much guaranteed in this type of situation,” said Phil Musser, a Republican strategist unaffiliated with any of the campaigns. He suggested that his party’s nominating contest was turning into a “multidimensional chess game.”

That is why so few of the leading candidates have engaged in any sustained assault or have run ad campaigns against Trump. It is a remarkable phenomenon: The party’s national front-runner remains largely unmolested as the first votes near, reflecting how his rivals believe that spending money against Trump and inviting his wrath could only help another hopeful.

Trump told reporters last week that, if attacked, he would hit back “very much and very hard in terms of ads,” but he has yet to reserve any advertising time.

The de facto nonaggression pact toward Trump — only Bush’s super PAC has confronted him, and it did so glancingly — is a sharp contrast from the last Republican presidential contest. In 2012, a super PAC supporting Mitt Romney spent millions airing harshly negative ads in Iowa against Newt Gingrich, the former House speaker, an assault that helped push Gingrich from first in the polls there to fourth when the caucus results were tallied.

But some of Trump’s rivals are privately skeptical that his support in the polls will translate into votes in February. And others believe that targeting him would only redound to benefit Cruz, who polls indicate is the leading second choice of Trump’s backers. So the other candidates are more focused on one another than on the dominant figure in the race.

“That’s part of the reason why you saw Bush and Christie go after Rubio,” said Carl Forti, a Republican strategist not tied to any of the campaigns. “They don’t gain much from hitting Trump at this point. Cruz is the one who immediately benefits from a Trump downfall.”

Cruz has steadfastly refused to take on Trump, who is closest to him in Iowa polls and atop every New Hampshire poll. And Cruz’s allies indicate that they are just as reluctant to target Trump, signaling that they have their sights set on Rubio, particularly his record on immigration.

“We have seen all year — and immovably so — that amnesty is a deal-breaker for voters,” said Kellyanne Conway, the head of a super PAC supporting Cruz, referring to the immigration overhaul bill that Rubio co-wrote in 2013. Conway said her super PAC, Keep the Promise, would begin airing television commercials this week in Iowa in addition to radio and digital advertising across the first four states that vote.

Cruz himself is bracing for an onslaught of attacks. “Strap on the full armor of God,” he told supporters in a conference call Thursday. “Come the month of January, we ain’t seen nothing yet.”

Santorum’s campaign was the first to directly confront Cruz on the air, going up with a commercial in Iowa mocking him for reading Dr. Seuss’ “Green Eggs and Ham” on the Senate floor. In addition to Santorum, the Iowa-based super PAC backing Huckabee is preparing to target Cruz more forcefully there. The super PAC supporting Bush, which this week started airing an attack on Rubio in Iowa, is running the same commercial in South Carolina, which votes third in the Republican contest.

But it is New Hampshire where the air war may be the most intense. The super PACs backing Bush and Kasich are already airing commercials there that gently criticize Christie (and, in the case of the pro-Bush group, Kasich as well). Rubio’s backers are likely to get in on the act in New Hampshire against Christie in January.

The super PAC backing Christie has nearly $7 million booked on broadcast outlets reaching New Hampshire voters, some of which has already been spent in the last month, and is prepared to go on the attack if it sees a threat.

In an interview last month in New Hampshire, Christie dismissed the coming assault on him.

“The guy with the most money has gone backwards in this race,” he said of Bush. “I don’t think paid media in this race matters nearly as much as it used to.”

As for the possibility that, once Republican voters are reminded of his vulnerabilities, his apparent revival could be snuffed out, Christie was just as brash. “I’m the most vetted guy in this race, so bring it on,” he said.

© 2016 The New York Times Company