Several GOP presidential candidates are starting to lay out their closing messages in a new round of campaign ads airing in Iowa and New Hampshire this week. The ads come ahead of the early February primaries in those states.

They strike a dark and fearful tone, with footage and news headlines highlighting the recent terror attacks in San Bernardino, Calif., and Paris.

Front-runner Donald Trump's campaign, in its first TV ad, begins with photos of President Obama and Hillary Clinton, followed by photos of the San Bernardino attackers laid over police lights. "The politicians can pretend it's something else, but Donald Trump calls it radical Islamic terrorism," a narrator says. "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."

"He'll quickly cut the head off ISIS, and take their oil."

The ad also highlights Trump's proposal to build a wall at the U.S.-Mexico border "that Mexico will pay for," over unidentified black-and-white footage of people running over a hill. (According to PolitiFact, the footage is from Morocco).

Watch the ad here:

Trump has so far relied on traditional media rather than TV ads, but the campaign will reportedly spend as much as $2 million per week on ads starting in the new year.

The candidate himself says he's not sure he needs it and that he's "probably wasting the money," but he wants to "be sure" in Iowa.

"I'm $35 million under budget," Trump said on CBS' Face the Nation Sunday. "Look, I was going to have 35 or 40 million spent by now. I haven't spent anything. I almost feel guilty. I think if you want to know the truth, I'm doing the ads. I'm leading by, as you all say, a lot."

Jeb Bush and his superPAC, on the other hand, have spent $38 million in campaign ads this election cycle, according to a count by NBC News. That spending dwarfs all other candidates in the race, including Democrat Hillary Clinton, whose campaign has spent about $12 million. But all that ad money hasn't helped Bush's lagging poll numbers.

Bush's new ad hits a dark and fearful tone, similar to Trump's. But he makes a not-so-subtle hit at the front-runner, as he attempts to portray himself as a more "serious" leader who will be able to take on the threat of terrorism.

The ad starts with a line from a recent speech: "Serious times require serious leadership." Bush's voice is laid over scenes of ISIS fighters, holding large black flags, explosions and headlines of recent terror attacks:

"In the absence of American leadership, the United States should not delay in leading a global coalition to take out ISIS with overwhelming force," Bush says. "We are at war with radical Islamic terrorism. We have but one choice: to defeat it."

Rick Santorum, lagging even further behind than Bush, also put out an ad Monday. His is aimed at Ted Cruz, who is polling ahead of Trump in Iowa in several polls, and focuses on Santorum's record in the Senate as well as the threat of ISIS.

"Ted Cruz is wonderful at reading children's fairy tales on the Senate floor," a narrator says, referring to the time Cruz read Green Eggs and Ham to his daughters during his 21-hour Obamacare filibuster in 2013.

"Rick Santorum spent his time in the Senate a little differently," the narrator says, highlighting his time on the Senate Armed Services Committee, "helping to modernize today's Army to better be prepared for today's threats."

The ad ends with a plea to voters strikingly similar to Bush's: "Serious times need serious people."

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