Likely voters in the Republican primaries put national security at the top of their list of concerns. Party strategists hope to use worries about the state of the world as a weapon against former Secretary of State Hillary Rodham Clinton if she becomes the Democratic presidential nominee.
But before they can do that effectively, Republicans need to agree among themselves on foreign and defense issues. Right now, they appear to be moving further apart.
Those deep and sometimes bitter divisions could hamper the party's eventual presidential nominee. For now, they are likely to prove a serious impediment to the aspirations of one of the chief actors in the drama over surveillance: Sen. Rand Paul of Kentucky.
Paul's strong attacks on the National Security Agency, as well as his championing of a less interventionist foreign policy, have helped him gain ardent supporters and a strong fundraising base as he pursues his party's presidential nomination. But those same issues have also sparked deep opposition to him among party leaders and may limit his ability to broaden his appeal among voters.
A poll released over the weekend of Iowans who are likely to take part in the state's January caucuses found that the share who rated Paul favorably had fallen 9 percentage points since last January, the largest drop of any of the Republican candidates. The Des Moines Register/Bloomberg Politics poll found that Paul had strong support among Republican voters younger than 45 but that his overall support had slipped in recent months.
Paul clashed openly Sunday in the Senate with his fellow Kentucky senator, Majority Leader Mitch McConnell, as well as with the 2008 Republican presidential nominee, Sen. John McCain of Arizona. McCain tried for a time to block Paul from speaking.
On Monday, the debate gained another voice as Sen. Lindsey Graham of South Carolina, a longtime McCain ally, formally entered the race for the GOP nomination with an announcement focused almost exclusively on foreign policy and defense issues.
Paul's supporters assert that his strong stands will help his candidacy, even with some voters who disagree with him.
"The Lindsey Grahams may gnash their teeth" over Paul's views, but voters may also see his position as one that comes from "a place of deep concern," said Jesse Benton, a longtime advisor to Paul. "I'm not so sure we don't benefit."
What is clear is that the debate has gained intensity, dividing the party on an issue — national security — that was once a source of electoral strength.
Graham, a retired Air Force officer who is one of his party's leading voices on defense, is positioned almost as the anti-Paul, defending the NSA at home and advocating an interventionist foreign policy abroad.
With little support in polls and a limited ability to raise the money needed to quickly elevate his national profile, Graham's quest for the nomination is clearly a long shot. But his home state presence in South Carolina, which has one of the earliest nominating contests, will probably give him the opportunity to press other candidates, particularly Paul, on security.
"Those who believe we can disengage from the world at large and be safe by leading from behind, vote for someone else. I am not your man," Graham said after a high school marching band in his hometown of Central, S.C., played the World War II-era favorite "Boogie Woogie Bugle Boy."
Americans hold divided views about anti-terrorism efforts. They have generally opposed the NSA's broad data collection activities that are intended to root out terrorist plots, but Republicans in particular have worried about whether the government is acting forcefully enough to prevent attacks.
Asked whether government security policies were going "too far in restricting civil liberties" or "not far enough in protecting the U.S.," Republicans by a nearly 2-1 ratio said in a January poll by the nonpartisan Pew Research Center that the government was not going far enough. Self-identified conservative Republicans held that view 54% to 31%.
That marked a large turnaround from two years ago, when conservatives were more likely to say they worried about the government infringing too much on civil liberties and when Paul's star within the party was on the rise.
In addition to the debate over anti-terrorism policies and surveillance, Republicans are divided over how to respond to the rapid rise of the Islamic State militant group in Syria and Iraq, as well as the legacy of the Iraq war.
Potential presidential candidates and party officials agree on criticizing the Obama administration — arguing that the president has been too passive in responding to the civil war in Syria and that he squandered U.S. gains in Iraq by not pushing harder to keep troops there. But they have less to say about what they would do differently.
Graham has advocated sending U.S. troops back to Iraq to bolster Iraqi units in the fight against Islamic State militants, who have seized control of much of the western part of the country as well as large chunks of neighboring Syria.
In his announcement Monday, he evoked former President Reagan's slogan of "peace through strength," which has long been a GOP mainstay.
He vowed to "take the fight" to the nation's enemies, particularly in the Middle East, and to "end this conflict on our terms."
"I want to be president to defeat the enemies that are trying to kill us," he said. "Security through strength will protect us."
Most other Republican presidential hopefuls have stopped short of advocating U.S. troops on the ground.
Jeb Bush, the former Florida governor, repeatedly stumbled last month over his thoughts on the war in Iraq ordered by his brother, President George W. Bush. In recent interviews, he has expressed skepticism about a renewed U.S. troop presence in the Mideast.
Sen. Marco Rubio (R-Fla.) has staked out a hawkish rhetorical position, comparing his approach to fighting militants with Liam Neeson's character in the film "Taken."
"Have you seen the movie?" Rubio asked at a GOP event last month in South Carolina, paraphrasing one of its signature lines: "We will look for you, we will find you and we will kill you." But Rubio has generally talked about air support, rather than a role for ground troops.
Paul, by contrast, recently accused Graham, McCain and other Republicans who support arming insurgent groups in Syria of contributing to the rise of the Islamic State group, also referred to as ISIS.
"ISIS exists and grew stronger because of the hawks in our party, who gave arms indiscriminately, and most of those arms were snatched up by ISIS," he said.
Graham has done little to hide his distaste for Paul's policies. Graham did not originate the "wacko birds" label for Paul and his allies — that was coined by McCain — but he might as well have.
Just over a week ago, Graham was caught on camera rolling his eyes as Paul objected to Senate efforts to continue the NSA's once-secret collection of records of Americans' telephone calls.
Paul returned the favor by releasing a video denouncing the NSA program that ridiculed Graham and Sen. Ted Cruz of Texas for their support of its surveillance, depicting Graham as trying to read Americans' email while sitting in a 1997-era car.