The Republican presidential candidates kicked off the 2016 election year on Monday with a burst of new attacks and advertising – including Donald Trump’s first TV ad of the season – as they entered the final sprint to Iowa and New Hampshire.

As Trump rolled out his TV spot in both early-voting states, Ben Carson released a new tax plan, just days after shaking up his struggling campaign. Ted Cruz was launching an aggressive 36-county tour across Iowa, while Marco Rubio used a security speech Monday in New Hampshire to slam those who voted to rein in America’s intelligence efforts.

The speeches, the ads, the bus tours and the proposals all reflect an effort by the campaigns in the still-crowded field to effectively re-launch, with less than a month to go until the lead-off Iowa caucuses, and after that the New Hampshire primary.

Carson, who has endured a precipitous drop in the polls and switched up his top campaign staff last week, unveiled a plan Monday to scrap the tax code and replace it with a 14.9 percent flat tax.

“No deductions, no loopholes -- it applies to everybody across the board,” the retired neurosurgeon told Fox News.

Once the darling of Iowa conservatives, Carson has seen his numbers plummet there as Cruz, the Texas senator, has shot to the front of the field – nudging past Trump.

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Though Trump still holds the lead in most New Hampshire polls, the billionaire businessman is seeking to energize his bid by putting money behind his first TV ad of the race. His campaign announced it would be spending at least $2 million a week on the ad, split between Iowa and New Hampshire.

The ad reprises Trump’s call to build a wall along the U.S.-Mexico border and to take the Islamic State’s oil, but begins by defending the controversial proposal he made after the San Bernardino terror attack to ban Muslims from entering the U.S.

The narrator in the ad says: “The politicians can pretend it’s something else but Donald Trump calls its radical Islamic terrorism. That’s why he’s calling for a temporary shutdown of Muslims entering the United States until we can figure out what’s going on.”

Trump isn’t the only one talking tough in the final stretch.

Florida Sen. Rubio, at an American Legion post in New Hampshire, took an implicit shot at candidates like Cruz in blasting lawmakers who voted to rein in the NSA – while vowing to “restore” such intelligence programs.

“If ISIS had lobbyists in Washington, they would have spent millions to support the anti-intelligence law that was just passed with the help of some Republicans now running for president,” Rubio charged.

He also blasted Democratic front-runner Hillary Clinton as “incompetent,” and said, “She has lied” – referring to her public explanations regarding the Benghazi terror attack. 

Keep the Promise, a super PAC supporting Cruz, launched its own ad in Iowa that mocked Rubio over a video clip he put out in October joking about “fantasy football” – the pro-Cruz ad juxtaposes that against images of ISIS fighters and the refugee crisis to question his seriousness.

Cruz, meanwhile, is keeping a packed schedule as he launches a six-day, 36-county bus tour in the Hawkeye State – and tries to maintain his lead there.

Even the lower-polling candidates are taking another crack at breaking through. Former Pennsylvania Sen. Rick Santorum, who won the Iowa contest in 2012, launched his first TV ad in the state on Monday – also going after Cruz.

The ad starts with a clip of the Texas senator reading a Dr. Seuss classic, "Green Eggs And Ham," on the Senate floor, which he did during a filibuster-like speech opposing ObamaCare in 2013.

"You want someone to read one helluva bedtime story, Ted Cruz is your guy," a narrator says. "If you want to protect America, and defeat ISIS, Rick Santorum’s your president. Because serious times need serious people."

Santorum, though, is polling at under 1 percent in the state as Cruz and Trump maintain an unrivaled lead.

New Hampshire, by contrast, is a much tighter contest. No fewer than five candidates are jockeying closely for the position behind Trump in the polls right now: Rubio, Cruz, New Jersey Gov. Chris Christie, Ohio Gov. John Kasich, and former Florida Gov. Jeb Bush.