Whom or what do the 2016 candidates consider America's greatest threat?

Despite economic issues being the primary focus of previous elections, the growth of terrorism abroad and, to some degree, at home, is making many Republican candidates shift the emphasis toward national security issues. With the myriad threats facing the United States, what do the candidates on both sides of the aisle consider to be the greatest threat to U.S. national security?

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"[Jihadists] are an existential threat to our nation. And we have to be mature enough to recognize that our children will have no future if we put our heads in the sand," retired neurosurgeon Ben Carson said at the second GOP debate in September.

During the fourth GOP debate, when asked by Fox Business Network's Maria Bartiromo what the biggest threat facing America is today, former Florida Gov. Jeb Bush replied, "I'd say it is Islamic terrorism, and, back to the question of what we are dealing with in Iraq, when we pull back, voids are filled. That's the lesson of history."

"[T]he single biggest national security threat facing America right now is the threat of a nuclear Iran," said Sen. Ted CruzRafael (Ted) Edward CruzO'Rourke prepping run for governor in Texas: report Support for Abbott plunging in Texas: poll White House debates vaccines for air travel MORE (Texas) at the September GOP presidential debate.

Current and former Democratic candidates echoed these sentiments during their presidential debates. Regarding what he considered to be the greatest threat to America, former Rhode Island Gov. Lincoln Chafee, who has since dropped out of the running, said, "It's certainly the chaos in the Middle East. There's no doubt about it."

"I believe that nuclear Iran remains the biggest threat, along with the threat of ISIL; climate change, of course," said former Maryland Gov. Martin O'Malley said when asked the same question, using an alternative acronym for the Islamic State of Iraq and Syria (ISIS).

Former Secretary of State Hillary ClintonHillary Diane Rodham ClintonClinton lawyer's indictment reveals 'bag of tricks' Attorney charged in Durham investigation pleads not guilty Attorney indicted on charge of lying to FBI as part of Durham investigation MORE replied similarly, saying the spread of nuclear weapons and nuclear materials presents a constant threat, especially if they should fall into the hands of terrorists that want to cause harm to the U.S.

When asked at the most recent Democratic debate in November if he still believed that climate change was the greatest threat to national security, independent Vermont Sen. Bernie SandersBernie SandersManchin suggests pausing talks on .5 trillion package until 2022: report Yarmuth and Clyburn suggest .5T package may be slimmed Sanders calls deadly Afghan drone strike 'unacceptable' MORE said, "Absolutely. In fact, climate change is directly related to the growth of terrorism."

For added context pertaining to how, and at an operational level, threats are assessed, when describing how he came to the conclusion that Russia is the top threat to the U.S. — several military officials expressed the same feeling to lawmakers in a spate of congressional hearings recently — Army Chief of Staff Gen. Mark Milley explained that it comes down to capability and intent. Concerning Russia's capabilities, he noted at the 2015 Defense One Summit that Russia is "the only country on Earth that has the capability to destroy the United States of America — it's an existential threat by definition because of their nuclear capabilities." Regarding Russia’s intent, while noting it is difficult to predict the intent of a nation or leader, past is prologue. "Since 2008, Russia [has] invaded sovereign territory of other countries in an aggressive manner. ... Their behavior is aggressive, their capability — nuclear capability — is significant, they've reorganized their conventional capability, their special operations capability, so Russia bears close watching."

The recent attacks in Paris claimed by ISIS changed the game in terms of what many believe the group is capable of beyond its safe havens in the Middle East and Africa (despite the fact that experts had warned previously that these types of attacks were not out of the realm of possibility). ISIS has always expressed the intention to hit the West, despite what some have said to the contrary. The group has demonstrated that it possesses this capability, which, along with the wider concern of its volatile ideology that inspires those to act on its behalf, elevates this threat. However, given ISIS's rise, focus has been primarily aimed at their actions, despite several other groups falling under the broad umbrella of radical Islamic terror. Al Qaeda's Yemen affiliate, for example — still considered to be the group's most volatile and capable — has significantly benefited from the domestic chaos brought on by the Saudi intervention since March 2015, allowing it to consolidate greater influence.

The threat from Iran does not presently rise to the level of other significant threats. Iran's nuclear capability is nascent, unlike that of other nations such as Russia or North Korea. However, Iran's behavior, such as the alleged ballistic missile test in violation of a U.N. Security Council resolution, as well as the threat it poses to U.S. interests and even personnel in the Middle East by way of its proxies and soldiers battling ISIS on the ground, is cause for concern.

Another threat that is seldom mentioned in debates or campaign rhetoric is that from the cyber domain. "Our greatest long-term strategic challenge is our relation with China. Our greatest day-to-day threat is cyber warfare against this country. Our greatest military-operational threat is resolving the situations in the Middle East," former Virginia Sen. Jim Webb, who bowed out in October, said at the first Democratic debate, providing a more holistic threat assessment than most other candidates.

What is different about the cyber domain is that it enables lesser actors to level the playing field, while enabling more capable actors and nation-states to enhance current capabilities. "[Y]ou can spend a little bit of money and a little bit of time and exploit some our weaknesses, and cause us to have to spend a lot of money, a lot of time," Defense Department CIO Terry Halvorsen said in September regarding the cost imbalance cyber affords.

Several officials have warned that a single cyber incident could take critical U.S. infrastructure offline and severely cripple the U.S. economy. However, despite the cost differential this space affords, non-state actors and small-time hackers still do not possess the capabilities to inflict a great deal of damage on U.S. networks or infrastructure. "Cyber attack by non-state terrorist groups is not a threat at this time. It takes a large, well-resourced, and time-intensive effort to use cyber tools for major disruption or physical damage," wrote James Lewis, senior fellow and program director at the Center for Strategic and International Studies, in a recent report. "China and Russia are world leaders in cyber capabilities for both espionage and attack. Their activities challenge all nations." Lewis also added that the most damaging cyberattacks that can cause physical damage to infrastructure are a "high art" and only a few nations possess this capability. "[I]t is likely that Russia has this capability, that China may already possess it, and Iran and North Korea are striving to acquire it and already have some capability to disrupt critical services."

The 2016 presidential election is posturing to be one focused primarily on national security and safety. The above list is by no means exhaustive, given the wide variety and seriousness of various threats. While the campaign season is still early, candidates have just skimmed the surface in terms of threats to the United States and many have not clearly articulated how they would respond as the new president.

Pomerleau is a freelance journalist based in Washington covering politics and policy. Follow him @MpoM24.