Last year, Islamic State seized control of big chunks of Iraq and Syria. This year, it’s hijacking the U.S. presidential campaign.

The biggest concern for most American voters until recently was the sluggish economy and stagnating incomes. But since the terrorist attacks in Paris and San Bernardino, more and more voters are saying national security is their primary worry.

A CNN Poll among Republican voters in Iowa last week found that 40 percent listed either terrorism or foreign policy as the top issue they would consider as they choose a candidate for president. Only 29 percent named the economy. Among Democrats, the effect has been less dramatic; if most Republicans are from Mars, most Democrats are still from Venus. The latter continue to list the economy above terrorism, although the margin has narrowed.

As a result, the attacks have changed the presidential campaign in both parties — from a contest focused almost solely on economic growth and tax rates to one that also includes questions about Muslim immigration and Islamic State.

That’s been good for some candidates and bad for others.

The losers have included Sen. Rand Paul, whose campaign for a more restrained foreign policy has fallen on increasingly deaf ears, and neurosurgeon Ben Carson, who struck many voters as ill-prepared in recent debates.

On the Democratic side, the spotlight on national security hasn’t been kind to Sen. Bernie Sanders, who says he wants to keep his focus on economic issues no matter what the polls say.

Among the winners — at least temporarily — has been frontrunner Donald Trump, who used the crisis to seize center stage last week by declaring that no Muslim should be allowed to enter the United States “until our country’s representatives can figure out what is going on.”

(Trump later clarified that he would allow Muslim U.S. citizens, including those in the armed forces, to return home despite their faith.)

Trump’s proposal drew denunciations from Sen. Marco Rubio (who called it “offensive”), Jeb Bush (“unhinged”) and House Speaker Paul Ryan (“This is not conservatism”) — but not Sen. Ted Cruz, who said he wanted to “commend” Trump for pointing out “the need to secure our borders.” Indeed, there is a genuine and intermittently serious debate about foreign policy among Republicans — one that Trump’s outbursts have mostly obscured.

Traditional GOP hawks, including Rubio, have said they favor deploying U.S. ground troops to Iraq and Syria, to serve as the nucleus of a multinational force aimed at defeating Islamic State on the ground.

Others, including Trump and Cruz, have suggested that unlimited U.S. bombing from the air should be sufficient. (Few, if any, military experts agree.) “We will utterly destroy ISIS. We will carpet bomb them into oblivion,” Cruz told voters in Iowa. “I don’t know if sand can glow in the dark, but we’re going to find out.”

Hillary Clinton has been sounding hawkish as well, noting that she has long favored a more assertive strategy in Syria than President Obama has pursued.

“We’re not winning,” she said on ABC News Dec. 5.

“Clearly we have to have a much more robust air campaign against ISIS targets, against the oil infrastructure, against their leadership … . I do believe we have to (increase) our special ops numbers. The 50 (troops) that have been authorized need to get there. And then we need to take stock of what else we need.”

That gung-ho tone hasn’t hurt her among Democratic voters. The CNN poll in Iowa found that 72 percent of Democrats — including some self-identified Sanders supporters — said Clinton would be their party’s best candidate to handle foreign policy.

In most years, a focus on national security and defense bolsters Republicans in a general election.

“If you ask voters which party they have confidence in on terrorism and defense, Republicans win that debate,” GOP pollster David Winston told me Tuesday. That’s one reason why George W. Bush won the 2004 presidential election against John F. Kerry, he said.

But this year, if the GOP nominates a candidate who’s unconvincing on foreign policy — or downright scary — that historical advantage could erode. Trump’s reckless xenophobia might yet work in a crowded primary; in a general election it’s more likely to lead his party to disaster.

“Who does it help?” Dana Perino, a former Bush aide, said of Trump’s rhetoric. “Hillary! That’s who.”

In 2008, when Clinton ran for the Democratic nomination against Obama, her campaign aired a famous television commercial that began with the sound of a ringing telephone.

“It’s 3 a.m. and your children are safe and asleep, but there’s a phone in the White House and it’s ringing,” a narrator said. “Who do you want answering the phone?”

If Clinton and Trump win their parties’ nominations, you can expect that commercial — or one like it — to make a return appearance.

— Doyle McManus is a columnist for the Los Angeles Times. His email address is doyle.mcmanus@latimes.com.