(CNN)Pressing to send a new wave of U.S. combat troops into Iraq would have amounted to political suicide for White House hopefuls just a few years ago.
GOP grapples with ground troops in Iraq
But the shocking advances of ISIS over the past year -- from the gruesome beheadings of Americans to the group's success in conquering key Iraqi cities like Ramadi -- are creating a new uncertainty about whether the U.S. should re-engage in Iraq, thrusting the issue to the fore of the 2016 presidential race.
The fear and alarm many Americans are feeling about ISIS is altering the political calculus for would-be GOP candidates, making a once perilous position on sending in ground troops more palatable, particularly among younger voters.
Few of the potential candidates have clearly defined their strategy for dealing with ISIS, and that is in part because they face a precarious balance: GOP candidates may win over Republican voters by embracing more hawkish views in the primary but they must avoid alienating moderate voters as they pivot toward the general election next year. A slender majority of Americans still oppose sending in ground troops. And many Democratic and independent voters are still uneasy about -- or outright opposed to -- the prospect of a renewed military campaign.
This week, as the White House weighed how much force to put behind what it called an "Iraqi-led operation to retake Ramadi," former New York Gov. George Pataki was the latest potential 2016 presidential candidate to argue for a more robust presence of U.S. ground troops to face ISIS.
He said that while he didn't want the United States to send "a million soldiers" and spend "a trillion dollars" trying to create a democracy in the region, he would "send in the troops, destroy their training centers, destroy their recruitment centers, destroy the area where they are looking to plan to attack us here and then get out."
Pataki was not as specific as South Carolina Sen. Lindsey Graham, who recently said he would send in 10,000 American troops to fight ISIS. And Pataki emphasized that his goal was not to "build a stable state" in the region, but rather to "protect our security interests here."
In Iowa on Wednesday, former Texas Gov. Rick Perry said eliminating ISIS would require "some boots on the ground."
"It certainly can be our allies' boots on the ground as well," Perry said.
On the other end of the spectrum within the GOP, Kentucky Sen. Rand Paul has said he believed the Iraq war was a mistake at the time of the 2003 invasion and that the ouster of Iraqi President Saddam Hussein did more harm than good for the region. Though Paul says he supports military action against ISIS, he has called for "Arab boots on the ground" rather than U.S. troops.
Reflecting the fraught politics of the issue, other potential GOP contenders have parsed their positions on ground troops carefully.
Former Florida Gov. Jeb Bush, who got tied up in knots last week as he tried to explain his position on his brother's invasion of Iraq, staked out a politically safe position on Wednesday in New Hampshire by saying he would defer to the advice of U.S. military commanders when it came to putting additional ground troops in Iraq. He also sought to steer the debate back to more comfortable ground: what he views as the Obama administration's failure to leave a residual force in Iraq.
Obama's decision to withdraw troops, Bush said in New Hampshire, was based on a campaign promise rather than conditions in Iraq at the time: "I think we're paying a price for it."
Many other GOP hopefuls, including Perry, Wisconsin Gov. Scott Walker and Florida Sen. Marco Rubio, have been vague about how far they would go to defeat ISIS -- stating that they would keep all options, including ground troops, on the table.
Kori Schake, a research fellow at the Hoover Institution at Stanford University who advised John McCain's campaign in 2008, noted that the GOP candidates are in a bind because Americans want contradictory things.
They want to defeat ISIS, said Schake, who worked on the National Security Council under President George W. Bush, "but they don't want the United States to have to fight that war and they don't want the United States to be involved in the messy political negotiations and state-building that will make for a long-term solution."
"What Republicans haven't yet been able to do is come up with a strategy of limited American involvement that manages to both pose a credible alternative" to Obama, she said, "and not to draw us in to a long-term and deep involvement in the war."
How far Republican candidates are willing to go on Iraq as they sketch out their positions over the next year will depend in large part on the level of anxiety among voters. For now, concern remains strongest among Republican voters.
In an April CNN-ORC poll, more than two-thirds of Americans -- 68% -- described ISIS as a very serious threat. But opinion was neatly divided on engagement, with 47% of respondents saying they supported the use of U.S. ground troops against ISIS in Iraq and Syria, and 50% saying they were opposed.
Sixty percent of Republicans polled said they were in favor of the use of ground troops, with 37% opposed.
Perhaps the most striking shift in public opinion has been among 18- to 29-year-olds who are becoming increasingly hawkish on the question of U.S. involvement. A Harvard University Institute of Politics survey released in late April showed that 57% of that group favored sending ground troops to participate in a military campaign against ISIS.
That's a dramatic shift from eight years ago when 60% of that age cohort said most or all troops should be withdrawn from Iraq.
"This youngest generation of first-time voters doesn't have a personal connection to 9/11, to Hurricane Katrina, to [George W.] Bush and then the promise of Obama" as they contemplate foreign policy for the first time, said John Della Volpe, director of polling at the Institute of Politics. "There's certainly an indication that the youngest voters are interested in a stronger, more forceful foreign policy."
He noted that in the April survey nearly a quarter of the 18- to 29-year-old cohort (23%) agreed with the premise of the so-called Bush doctrine that it is sometimes necessary to attack potentially hostile countries, rather than waiting until the United States is attacked to respond. That was a 7-point jump in the percentage of young voters holding that view from 2014.
Della Volpe estimated 20% of all voters will be under 30 in 2016. While the Republican candidate is unlikely to outright win that demographic, there are clear signs they could increase their share over past elections, he said.
There is frustration among Americans that ISIS remains so strong, but there does not yet appear to be a political consensus about what should be done to defeat them.
James F. Jeffrey, the former U.S. ambassador in Iraq, said there is a range of additional options available to U.S. leaders to go after ISIS: a model based on that in Afghanistan, where U.S. troops are accompanying, training and facilitating Afghan troops in the fight, a plan that involves more active raiding like the recent operations in Syria, or a third option putting an American battalion on the ground to seize and hold ground from ISIS.
But Republicans are having difficulty answering the question of what they would do, he said, in part because they must first answer a more basic question -- whether the goal is to defeat ISIS and what their timeline would be for doing so. Obama's policy, he argued, has essentially amounted to containment because the timeline is open-ended.
"The question is what is the mission that we want to accomplish in Iraq?" said Jeffrey, the Philip Solondz distinguished visiting fellow at the Washington Institute for Near East Policy.
At the political level, he said, all the candidates, including Democratic presidential front-runner Hillary Rodham Clinton, will have to clarify whether they are committed to destroying ISIS, not just containing it, and whether they are willing to go the lengths necessary to achieve that.