It wasn't long ago that the Kentucky senator seemed perfectly matched to the times: a Republican who reflected the nation's reluctance to keep launching wars but clung to a stinging critique of an intrusive, dysfunctional Big Government.

But for Paul, who formally announced his presidential campaign Tuesday, that balancing act may not be enough to build a coalition that can carry him to the White House. A cascading list of horrifying headlines from abroad is convincing many Americans that the national interest again requires the United States to project power in the chaotic Middle East.

In particular, the gruesome actions of ISIS -- including beheading Americans -- could undercut the anti-interventionist philosophy that is central to Paul's political persona.

'More difficult'

"From the end of the Bush years up through 2013 and '14, there was a war weariness that was permeating even Republicans," said David Boaz, author of a new book "The Libertarian Mind." "However, the videos of ISIS beheading Americans and other people have certainly made it more difficult to stick to a noninterventionist argument."

After doing penance after the debacle in Iraq, it suddenly seems acceptable for the GOP to be hawkish again. Republicans like Jeb Bush and Ted Cruz have lacerated the Obama administration's national security policy and managed to get to Paul's right on national security.

"The anti-interventionist wave I do think has crested and those issues are moving away from Rand Paul now," said Fergus Cullen, the former chairman of the New Hampshire Republican Party.

The change in the public mood is reflected in Paul's polling. A year ago, he led the Republican field in a CNN/ORC International Poll with 16%. In the latest survey, he was back in third place at 12 points. That same poll in March also reflected the level of public concern about ISIS. Eighty percent of those polled thought the organization posed a serious threat to the United Sates. And a similar percentage were worried that current military action against ISIS could develop into a wider war.

"The Republican Party, because of the rise of ISIS, because of national security becoming so prominent ... has become much more hawkish," said Kevin Madden, a former spokesman for GOP nominee for Mitt Romney. Paul "has a resume of statements and a resume of positions that might be out of step with many national security conservatives."

The key for Paul will be whether he has the political dexterity to build a bridge from his previous positions to where the party is now.

Clues

His announcement speech was full of clues about how he plans to do it as he moved hard to the right on national security, but with a caveat that he plans to steer clear of foreign quagmires.

"The enemy is radical Islam and not only will I name the enemy, I will do whatever it takes to defend America from these haters of mankind," Paul said. "I envision an America with a national defense unparalleled, undefeatable, and unencumbered by overseas nation building."

Photos: Rand Paul's political life U.S. Sen. Rand Paul delivers remarks while announcing his candidacy for the Republican presidential nomination during an event in Louisville, Kentucky, on Tuesday, April 7. Hide Caption 1 of 20 Photos: Rand Paul's political life Paul and his wife, Kelley, wave to supporters in Louisville on April 7. Hide Caption 2 of 20 Photos: Rand Paul's political life Paul speaks in Rochester, New Hampshire, prior to meeting potential voters in March. Hide Caption 3 of 20 Photos: Rand Paul's political life Paul walks on stage before speaking at the Values Voter Summit in Washington in September. Hide Caption 4 of 20 Photos: Rand Paul's political life Paul attends a Senate Foreign Relations Committee hearing on Syria in September. Hide Caption 5 of 20 Photos: Rand Paul's political life Paul speaks at a news conference in June about the U.S. role in Syria. Hide Caption 6 of 20 Photos: Rand Paul's political life Speaking at the University of California at Berkeley in March 2014, Paul speaks on the issues of privacy and curtailing domestic surveillance. Hide Caption 7 of 20 Photos: Rand Paul's political life Paul addresses the 2014 Conservative Political Action Conference, where he easily won the presidential straw poll. Hide Caption 8 of 20 Photos: Rand Paul's political life From left, U.S. Sens. Cory Booker, Kirsten Gillibrand and Paul attend a March 2014 news conference to announce a new medical marijuana bill at the U.S. Capitol in Washington. Hide Caption 9 of 20 Photos: Rand Paul's political life In February 2014, Paul announced that he was suing President Barack Obama and top national security officials over the government's electronic surveillance program made public by intelligence leaker Edward Snowden. Hide Caption 10 of 20 Photos: Rand Paul's political life Paul leaves the Capitol in March 2013 after staging a filibuster over the nomination of CIA Director John Brennan. Hide Caption 11 of 20 Photos: Rand Paul's political life Ethics laws prevent senators from accepting payment for any job other that their duties in Washington. So Paul, a veteran eye surgeon, performs pro-bono procedures to keep current on his practice. Hide Caption 12 of 20 Photos: Rand Paul's political life Paul speaks in Bowie, Maryland, in March 2013 during a discussion on reforming the criminal justice system. Hide Caption 13 of 20 Photos: Rand Paul's political life Paul and his father speak with supporters in Ames, Iowa, before the start of a 2012 presidential campaign event. Hide Caption 14 of 20 Photos: Rand Paul's political life Paul boards an elevator after attending a Republican caucus meeting in Washington in July 2011. Hide Caption 15 of 20 Photos: Rand Paul's political life Paul unveils his own version of the federal budget during a news conference in March 2011. Hide Caption 16 of 20 Photos: Rand Paul's political life Paul speaks to hometown supporters in Bowling Green, Kentucky, during his campaign for the Senate in November 2010. Hide Caption 17 of 20 Photos: Rand Paul's political life Paul and his family celebrate his 2010 Senate victory during an election night party in Bowling Green. Hide Caption 18 of 20 Photos: Rand Paul's political life Coming from his son's soccer game, Paul wears shorts and a suit jacket while preparing for his guest spot on a Fox News television program in May 2010. Hide Caption 19 of 20 Photos: Rand Paul's political life Paul waits to tour a mine in Pineville, Kentucky, in May 2010. Hide Caption 20 of 20

Paul's speech was also an implicit admission that he must build a more durable coalition in his run for president than the one that has sustained him in his single Senate term.

He sought to keep libertarians aboard with paeans to liberty and the Constitution. But Paul also aligned himself with Republican hawks by articulating a tough message on the Iran nuclear deal. And he tried to appeal to minorities by bemoaning the "daily ugliness that dashes hope" in inner cities and working-class whites by slamming income inequality.

His approach suggests that trouble with his political brand isn't just limited to foreign policy.

The tea party wave that the accomplished eye surgeon expertly surfed to the Senate six years ago is not the force it once was.

While there's always an opening for an assault on the size of government in the Republican primary, such a tack hardly differentiates Paul from his rivals. The improving economy, meanwhile, has slaked some of the fury about government bailouts and stimulus plans he used to such effect to confound the establishment and win in Kentucky.

"I think that clearly some of the tea party wave and some of that enthusiasm has waned somewhat," said Dewey Clayton, a professor of political science from the University of Louisville who has studied Paul's rise.

Timing

The shift is a reminder that timing is everything in presidential politics.

Successful campaigns are akin to catching lightning in a bottle -- and represent a rare convergence between a candidate's personality, policy ideas and the political demands of a unique moment in time.

Obama managed it in 2008, when his hope and change mantra captivated a nation weary of war and fearing economic collapse. Likewise, in 1980, Ronald Reagan's certitude in America's goodness wooed voters tired of U.S. humiliation abroad and economic stagnation under Jimmy Carter.

But it's just as possible that the moment in history when a candidate with Paul's potential and profile could win came during the anti-Big Government backlash that took place at a time of huge bank bailouts and foreign military excursions -- when he was not yet a national figure.

Of course, another key ingredient in White House campaigns is political skill. Paul has shown that he's got a talent for capturing the political Zeitgeist -- with a 13-hour Senate filibuster on drone strikes and by working on issues like drug sentencing and criminal justice other Republicans barely touch. And the first two GOP nominating contests look like a good fit for someone who leans libertarian.

Paul has come a long way from the Wall Street Journal article he penned in June 2014 when he warned America should not get involved in Iraq's civil war and asked "what would airstrikes accomplish?"

Even by September, that position was untenable for a potential presidential candidate and Paul began to argue that ISIS "required destruction."

Criticism from rivals

Such a shift from Paul, who enjoys a reputation as a politician who takes stands based on principle, not politics, did not go unnoticed by his GOP rivals.

"What is unfortunate, is that too many leaders in both parties, including our President and some who aspire to be president, have shown they would rather wait for poll numbers to change than demonstrate the leadership necessary to shape them," Florida Sen. Marco Rubio said in a national security speech last September.

In many ways, Paul's campaign will be an exercise in proving that a limited U.S. footprint overseas is still what voters want -- though actions by ISIS may mean the American appetite for overseas action may be more robust than he originally foresaw.

But it is not all about national security.

His campaign also represents the most authentic test yet of whether libertarian politics is ready for the mainstream.

No one would argue that Paul, who has positions in line with social conservatives on same-sex marriage and abortion, is a straight down the line libertarian.

But his support for low taxes, a "restrained" IRS, plans to eliminate a string of government departments and privacy rights sit clearly in the libertarian creed.

"He is going to offer the most libertarian policy platform of any major presidential candidate in memory," said Boaz, adding that previous libertarian presidential candidates like Gary Johnson and Ron Paul were more classically libertarian but had no chance of the nomination.

Boaz says there may be a larger potential pool of Rand Paul voters than many analysts think. He points to the 2014 Governance Survey by Gallup, which finds that 24% of respondents could be characterized as libertarians.

A Zogby poll found that 44% of voters defined themselves as "fiscally conservative and socially liberal" and while they may not classify themselves as libertarians, they could at least give Paul a hearing.

But Paul also knows -- and the 2008 and 2012 campaigns by his father Ron Paul prove it -- that relying on the libertarian base will not be enough to win the nomination.

That's why efforts he's already made to reach out to minorities and younger voters will be so crucial in what is shaping up as an intensely competitive race.