WASHINGTON -- Republican Mitt Romney accused President Barack Obama of "failing American workers" by ignoring Chinese trade violations, and seized on new Federal Reserve attempts to boost the economy as proof the administration's policies are not working.

The Republican candidate shifted his focus back to more comfortable ground on Thursday -- the struggling American economy -- after a one-day campaign detour into a foreign-policy thicket that left him bruised and Obama largely unscathed.

Romney made little mention during the day of the events in Egypt and Libya that he had cited Tuesday as evidence of national security weakness on the president's part. Democrats have derided Romney's remarks as inaccurate and evidence that the Republican nominee is an unsteady international leader. Even some Republicans said Romney spoke too hastily, although others defended him.

The economy has long been Romney's strongest area, with polls suggesting a majority of voters consider the multi-millionaire businessman better qualified than Obama to put the U.S. back on the road to prosperity. With the unemployment rate hovering stubbornly above 8 per cent, the economy is also the No. 1 issue for voters.

The debate over the Middle East turmoil intruded, though, when a heckler at Romney's rally yelled out, "Why are you politicizing Libya?" The crowd responded with chants of "U-S-A" and supporters tried to place a Romney placard in front of the heckler's face.

"We're going to crack down on China," the former Massachusetts governor vowed in an appearance in the Virginia suburbs around Washington, D.C. He spoke after his campaign unveiled a television commercial claiming that China has outpaced the United States in new manufacturing jobs since the president took office. "Seven times Obama could have stopped China's cheating. Seven times he refused," it says.

The president pushed back.

White House spokesman Jay Carney told reporters that all the actions the administration has initiated at the World Trade Organization to rein in China have been successful. The president's campaign said Obama has brought as many cases challenging China trade policies in 3 1/2 years as former President George W. Bush did in eight.

Obama campaigned as commander in chief after the violent deaths of four U.S. officials at a diplomatic post in Libya. "No act of terror will go unpunished ... no act of violence shakes the resolve of the United States of America," he said.

Obama vowed to do "whatever is necessary" to protect Americans serving abroad.

"We are going to bring those who killed our fellow Americans to justice," he said in Golden, Colorado as two U.S. warships head for the Libyan coast.

Halfway around the world, anti-American protests spread to Yemen.

Obama said the U.S. would not consider Egypt an ally, "but we don't consider them an enemy."

The government in Cairo receives roughly $1.5 billion in U.S. aid annually, most of it for the military.

The president said in an interview with the Spanish-language network Telemundo that Egypt is a "new government that is trying to find its way." And he warned that if the Egyptian government takes actions showing "they're not taking responsibility," then it would "be a real big problem."

Administration officials later said the president was not trying to downgrade the relationship between the U.S. and Egypt.

The candidates spoke with less than eight weeks remaining in a close campaign for the White House in tough economic times. The two states are among a handful likely to settle the race, and most polls rate Obama a shaky favourite. The U.S. president is chosen not by popular vote but in state-by-state contests.

Inevitably, the Fed's new attempt to intervene in the economy became enmeshed in the campaign.

The nation's central bank said it will spend $40 billion a month to buy mortgage bonds for as long as it deems necessary to make home buying more affordable. It plans to keep short-term interest rates at record lows through mid-2015 -- six months longer than previously planned -- and made clear it's ready to try other measures to stimulate the economy if hiring doesn't improve.

"The idea is to quicken the recovery," said Fed Chairman Ben Bernanke at a news conference where he announced the latest attempts to jolt a slow-growth economy that has left joblessness at 8.1 per cent.

Carney, the White House press secretary, declined to comment, citing a long-standing policy when it comes to Fed actions.

Not so Romney's campaign.

It released a statement in the name of Lanhee Chen, campaign policy director, that called the Fed's steps "further confirmation that President Obama's policies have not worked. After four years of stagnant growth, falling incomes, rising costs and persistently high unemployment, the American economy doesn't need more artificial and ineffective measures. We should be creating wealth, not printing dollars."

Romney, who has said he would not reappoint Bernanke to a new term, declined to respond to a question about the Fed's action. He has said previously that he opposes more measures along the lines that Bernanke announced during the day, but the written statement stopped short of saying so.