WASHINGTON - U.S. President Barack Obama's re-election campaign has opened an assault on challenger Mitt Romney's foreign policy, with Vice-President Joe Biden saying the presumptive Republican nominee's foreign policy vision is stuck in the Cold War era.

With Romney all but certain to challenge Obama in November, the president's campaign machine released a video Thursday that draws sharp contrasts between the candidates' views of U.S. relations abroad.

The video served as a preview to a speech Biden delivered in New York that paints the foreign policy outlined by Romney as shallow, ill-informed and dangerous. Biden said Obama will gladly stack accomplishments such as killing terror mastermind Osama bin Laden against Romney's rhetoric.

Romney advisers held a conference call before Biden spoke, with John Lehman, Navy secretary during the Reagan administration, accusing Obama of "a gross abdication of leadership" that could have practical and political consequences.

Until now, neither Romney nor Obama had made foreign policy a major issue in the campaign.

The Obama campaign also announced the president's first official re-election rallies next week, back-to-back appearances May 5 at Ohio State University and Virginia Commonwealth University. He will be joined by his wife, the popular Michelle Obama.

The campaign revved up two days after Romney's sweep of five Republican presidential primaries that virtually assured he would be the party's nominee. Those victories forced Newt Gingrich, former speaker of the House of Representatives, to finally acknowledge his campaign was no longer viable. Romney's most significant opponent, former Pennsylvania Sen. Rick Santorum, left the contest two weeks ago. Only Texas Rep. Ron Paul, a libertarian with a small but loyal following, is maintaining a token campaign.

Obama plans to use his return to campaigning to draw a contrast between his economic approach and what the campaign says is the Republican Party's desire to return to the policies that crashed the economy.

The economy is by far the biggest issue in this election.

News of Obama's first campaign rallies followed word from the Republican National Committee that it had filed a formal complaint with the Government Accountability Office requesting an investigation into whether Obama was using taxpayer money to fund travel that benefited his re-election campaign.

Party Chairman Reince Priebus said in a statement that Obama's campaign "has been cheating the American taxpayer by using taxpayer dollars to fund their general election efforts." That is a perennial complaint of opposition candidates when running against a sitting president.

Obama has spent the past two days courting young voters who were solidly behind him in the 2008 election. In 2008, he had a 34-point advantage over Republican challenger John McCain among voters under age 30. New polling suggests the president may face a harder sales job with younger voters this time.

Romney, meanwhile, moved aggressively to raise money for the battle against Obama and draw the support of conservatives within the Republican Party who continue to be skeptical of his positions.

Romney was attending fundraisers Thursday to prepare for what may be the most expensive presidential contest in the history of U.S. politics. He will have attended six closed-door fundraisers at the close Thursday of a two-day swing through New York and New Jersey.