WASHINGTON – For more than three decades, the Republican Party brand has been deeply tied to a worldview in which the aggressive use of American power abroad is both a policy imperative and a political advantage.

Now, a new generation of Republicans like Sen. Rand Paul of Kentucky is turning inward, questioning the approach that reached its fullest expression after the terrorist attacks of Sept. 11, 2001, and signaling a willingness to pare back the military budgets that made it all possible.

That holds the potential to threaten two wings of a Republican national security establishment that have been warring for decades: the internationalists who held sway under the elder President George Bush and the neoconservatives who led the country to long and costly wars in Iraq and Afghanistan under President George W. Bush.

Members of both camps said this week that they fear returning to a minimalist foreign policy, as articulated in different ways by Paul, Sen. Mike Lee of Utah and Rep. Justin Amash of Michigan.

The foreign policy hawks fear it would lead to a diminished role for America in an increasingly unstable world. And they worry about their party losing its firm grasp of what has traditionally been a winning issue.

"A real challenge for the Republicans as they approach 2016 is: What will be their brand?" said Richard N. Haass, the president of the Council on Foreign Relations and a former aide to the first President Bush. "The reason Rand Paul is gaining traction is overreaching in Iraq. What he is articulating represents an alternative to both."

The split in the party was on display in muted terms on Thursday at the opening session of the Conservative Political Action Conference when Sen. Marco Rubio, R-Fla., and a possible presidential candidate in 2016, expressed concern about a return to isolationism.

Without mentioning Paul directly, Rubio said that the United States "can't solve every war" but added that "we also can't be retreating from the world."

Moments later, Paul told the conference that the filibuster he conducted last week over the Obama administration's drone policy was aimed at the limits on presidential power and American power abroad.

"No one person gets to decide the law," he said.

Some Republicans are so nervous about the positions championed by Paul and his supporters that they have begun talking about organizing to beat back primary challenges from what Dan Senor, a veteran of the younger Bush's team of foreign policy advisers, described as a push to reorient the party toward a "neo-isolationist" foreign policy.

That policy, Senor said, "is sparking discussions among conservative donors, activists and policy wonks about creating a political network to support internationalist Republicans."

But in Paul and the tea party, Republicans face a philosophical disagreement from within their ranks.

Sen. John McCain of Arizona, who is his party's most prominent spokesman for an aggressive foreign policy, recently dismissed Paul and those who agree with him as "wacko birds."

But other party leaders are rushing to embrace Paul and tea party Republicans as they build coalitions of young voters who dislike the foreign wars and the cost of fighting them. Those voters may be a key to winning back the White House in 2016.

After Paul's 13-hour filibuster last week, leading Republican figures heaped praise on the freshman senator.

Sen. Ted Cruz of Texas and Lee joined the filibuster, offering their ideological support for his cause.

Paul calls himself a "realist, not a neoconservative nor an isolationist."

But his view of America resembles that of his father, former Rep. Ron Paul, who built a deeply committed following of libertarians and tea party Republicans by opposing most American involvement overseas.

Rand Paul, who is mulling a presidential bid in 2016, is less strident and more subtle than his father. In a speech at the Heritage Foundation last month, he insisted he is not against all foreign intervention but pledged to fight for "a saner, more balanced approach to foreign policy."

The question for the Republican Party is whether Paul and his followers will emerge as a vocal enough part of the Republican electorate to reshape the party's foreign policy without taking it back to the strictly isolationist approach.

"This is a divide that has been festering and deepening for a generation," said Thomas Donnelly, a fellow at the American Enterprise Institute, a conservative policy group. "It's bad for the country, bad for the party in a whole host of ways – particularly in presidential elections."

Some Republicans are less worried. They view Paul's crusade as nothing more than the usual attempt by members of the opposition party to undermine the assertive foreign policy of an incumbent president.

In the 1980s, Democrats harshly criticized President Ronald Reagan's attempts to arm Nicaraguan rebels. During the 1990s, Republicans derisively called President Bill Clinton's intervention in Kosovo "Clinton's war."

In Obama's first term, critics assailed his expansion of the war against terrorism, including the expanded use of drones.

"The last three presidents have worried about a rising tide of isolationism," said Peter D. Feaver, a professor of political science at Duke University who served as a national security aide for both Clinton and the younger Bush. "Sometimes it's the protectionist sentiments among Democrats. Sometimes it's the libertarian, extreme wing of the Republican Party. Sometimes it's just war fatigue."

Feaver said that many Republicans who praised Paul do not share his broader views about a limited role for the United States abroad.

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