Republican presidential front-runner Mitt Romney is preparing to broaden his challenge to President Obama’s management of foreign affairs, sensing political vulnerability in an area in which the incumbent has received his strongest public support.

With the nation facing a high unemployment rate and uncertain economic growth, foreign policy as a political issue has remained on the periphery of the presidential race. Republicans seeking their party nomination have trained their sharpest criticism of Obama on his economic record, where they perceive him to be weakest.

That is changing, particularly in the Romney campaign, as the former Massachusetts governor begins to set his sights on the general election. Advisers say Romney, cast by the Obama campaign as a foreign policy novice, is unwilling to concede the issue in what both sides say would be a close race.

The emergence of foreign policy in the campaign comes during a particularly difficult period in Afghanistan and at a time when fears are rising about Iran’s nuclear program.

Those concerns, and new ones about what Obama intends to do overseas if reelected, have elevated the issue in the race. Advisers say Romney intends to deliver a major foreign policy address in April or May, depending on the status of the primary contest, and create what one adviser described as a series of “platforms” to highlight the differences between the two candidates.

“In the end, this president will lose because he failed on the economy — that’s the referendum,” said Richard S. Williamson, a senior foreign policy adviser to the Romney campaign. “But the same sort of naivete and fecklessness have been evident on the foreign policy side as well. And we welcome having that debate.”

Advised primarily by veterans of the Reagan and both Bush administrations, Romney has been bolder in recent weeks in contrasting his foreign policy views with those of Obama.

In his speeches, Romney has proposed a more confrontational approach to China, Russia, Iran and other countries, one that would clearly identify the United States’ friends and enemies and treat them accordingly. He has also used blunt, swaggering language on the stump that at times has evoked the Cold War era, including his pledge to “devote myself to an American century.”

“And,” he said in his seminal foreign policy speech at The Citadel last October, “I will never, ever apologize for America.” On Thursday, former president George H.W. Bush, who made a similar pledge as vice president, endorsed Romney’s candidacy.

The political opportunity Romney sees in foreign policy was reflected this week when he seized on Obama’s open-mike conversation with his Russian counterpart, Dmitry Medvedev. In what he thought was a private exchange, Obama asked for more “space” during his reelection campaign that, if successful, would allow him to be more “flexible” in addressing Russia’s concerns during his next term.

Romney said Obama “signaled that he’s going to cave to Russia,” calling the country the United States’ “number one geopolitical foe.” He followed that up with an op-ed in Foreign Policy magazine, the title of which contended that Obama’s “ ‘hot mic’ diplomacy is endangering America.”

In recent weeks, Romney also has described Obama’s policy to stop Iran’s nuclear enrichment program as a failure, criticized his defense budget as cutting too deeply at a time of rising international threats, and hammered home his contention that the president has “thrown Israel under the bus” by pressuring its leaders to return to the peace table with the Palestinians.

Obama and his advisers say they welcome the debate and believe that the president’s positions are far more in line with public opinion than Romney’s, which they often characterize as anachronistic.

Asked Thursday about Romney’s characterization of Russia, White House press secretary Jay Carney told reporters: “I’m pretty sure the Cold War ended when some of the folks in this room were still in elementary school. And any suggestion that Russia is America’s number one geopolitical foe represents a profound — or unique — understanding of recent history.”

Other Republicans are speaking up on foreign policy, too. In a speech Thursday in California, former senator Rick Santorum (Pa.) said that Obama’s policies are “destructive for our economy, destructive for our reputation around the world and for our national security.”

Much of Obama’s foreign policy, which has emphasized collective action, has been a response to that of his predecessor, George W. Bush, who after the Sept. 11, 2001, attacks sent U.S. forces into two Muslim nations and who alienated some traditional U.S. allies with a largely unilateral approach.

Foreign policy analysts say Romney, too, has been forced to confront the Bush legacy, but within his own party. The wars in Afghanistan and in Iraq have created a strong isolationist strain within the Republican Party, embodied this campaign season by the candidacy of Ron Paul.

At the same time, another wing of the party argues that the United States must still play the leading role in the world on behalf of democracy, human rights and open markets — “an exceptional country with a unique destiny and role in the world,” as Romney put it at The Citadel last fall.

“What Romney is trying to do, in the midst of a campaign, is to bring together these different strands of the Republican constituency that seem to have fractured after the Bush presidency,” said Jon B. Alterman, who holds the Zbigniew Brzezinski chair in global security and geostrategy at the Center for Strategic and International Studies.

But, Alterman said, Romney is attempting to “criticize the current foreign policy without a well-thought-through set of alternatives going forward.”

He said budget constraints, which prompted Obama to undertake a major defense policy review with his senior combat commanders late last year, will severely complicate the next commander in chief’s job.

Romney “must answer the question: ‘What are you willing not to do anymore?’ ” Alterman said.

In public opinion polls, Obama has scored higher on foreign policy issues than any other candidate.

He approved the mission in May that killed Osama bin Laden, used U.S. forces to help topple Moammar Gaddafi, and recommitted the United States to working through international organizations such as the United Nations and NATO.

In other areas, including U.S. war policy in Iraq and Afghanistan, the public favors Obama’s position far more than those of his chief Republican rivals.

While the Obama administration has trimmed back its goals in Afghanistan — putting the focus on defeating al-Qaeda instead of the Taliban — Romney has said he would closely follow the advice of battlefield commanders in setting troop levels there. He has declared that the Taliban should be defeated to ensure the country’s stability once U.S. forces leave.

More recently, he has criticized Obama for setting a timeline to bring home troops who were part of the “surge” into Afghanistan in 2010, saying doing so allows the Taliban to just wait out the war.

But a New York Times-CBS News poll published this week found that 69 percent of Americans think the United States should no longer be at war in Afghanistan — the highest level of opposition to date.

Obama and Romney also disagree on the endgame in Iraq, a war Obama once called “dumb.” Romney has criticized Obama for not securing an agreement with Iraqi political leaders to maintain a U.S. troop presence in the country, which is still riven by ethnic and sectarian animosities.

“There are clearly some differences between this president and the other side,” said Jim Messina, who is managing Obama’s reelection campaign. “The major Republican candidates wanted us to stay in Iraq longer; Mitt Romney wants us to have a longer engagement in Afghanistan. Those are differences.”

But Obama could be vulnerable in some areas. He has sometimes seemed overly cautious in managing the widespread changes recasting the Middle East as a result of the Arab Spring, preferring to allow European countries to lead in a part of the world where American support can prove problematic.

He also has not been able to revive the moribund Israeli-Palestinian peace talks and to deter Iran from its uranium enrichment program, which Israel thinks will be used to build a nuclear weapon.

“The world is better off with Osama bin Laden dead and Moammar Gaddafi dead,” said Williamson, the Romney adviser. “But two deaths do not a foreign policy make. If the president’s campaign suggests he will get a free pass, I would suggest to you they are sorely mistaken.”

Polling analyst Scott Clement and staff writer Felicia Sonmez contributed to this report.