The presidential candidates grasped for momentum Thursday in the closing days of a hard-fought race, with President Obama returning to the campaign trail after a three-day hurricane hiatus while Mitt Romney launched a last-minute effort to snatch away vote-rich Pennsylvania.

The battle for the White House, which extended across four time zones amid a flurry of new advertisements and attacks, marked the lead up to the final four days of a contest as close as any in recent history. The latest Washington Post-ABC News tracking poll showed the candidates within decimal points of each other nationally and across the battleground states.

The president welcomed the surprise endorsement on Thursday of New York Mayor Michael R. Bloomberg, a high-profile independent who had recently appeared likely to sit out the election. Independents have been trending toward Romney in recent polling. Obama’s campaign also released a television ad hailing the endorsement of retired Gen. Colin L. Powell, who worked in several Republican administrations.

Obama’s handling of Hurricane Sandy has won him plaudits, including from Republican Govs. Chris Christie of New Jersey and Robert F. McDonnell of Virginia, both top Romney surrogates. But Romney was not deterred, pressing his argument during three rallies in Virginia that his business experience could turn around the sluggish economy.

With the candidates seeking anything to move the needle, both campaigns looked ahead to a possible wild card: the release of Friday’s unemployment report, especially key in a campaign that has turned on the economy. The two sides plan a frenetic weekend of travel, with Obama blitzing four battleground states alone on Saturday and Romney holding a massive evening rally on Friday in Ohio with nearly 100 top surrogates.

The presidential campaign is heading into its final days, and the contest remains deadlocked nationally.

The contest has been slowly returning to normal after the storm battered the East Coast this week. Romney resumed a full schedule of events on Wednesday, although he briefly left it to surrogates to attack the president. Obama, in full commander-in-chief mode, had spent three days immersed in briefings and travel as he helped manage the federal response to the hurricane.

Even as the storm’s death toll rose, the political broadsides resumed in full force on Thursday. And Obama, who campaigned on the theme of change four years ago but has struggled to land on a simple reelection message, returned to his core argument, telling voters in far-flung time zones that Romney would turn the clock of progress backward. “We know what change looks like. What the governor is offering sure ain’t change,” Obama said at a morning event. At another event hours later, he argued: “I know what real change looks like, because I fought for it.”

Romney has been claiming the mantle of change as well. Late Thursday, a senior adviser said the Republican nominee will begin the final four days of the campaign with a speech in West Allis, Wis., “to make clear the big choice voters have to bring about real change.”

Even as Obama flew almost all the way across the country, from Wisconsin to Nevada to Colorado and then to Ohio for several hours of sleep — and as Romney spent the day campaigning in Virginia — the focus of both campaigns’ ground games stayed locked on Ohio.

Obama’s campaign used a television ad and a conference call to attack a controversial Romney ad that criticized the automobile-industry bailout the president oversaw. The bailout particularly resonates in Ohio and in Michigan, a traditionally blue state in which Romney has been making tentative inroads. Polling suggests the race has tightened over the past month, including in Ohio, even as Obama retains the slightest of edges.

In another tweak of the map, Romney announced plans to make a full-fledged effort in the traditionally Democratic state of Pennsylvania. Trying to expand an electoral map that most analysts think is more difficult than Obama’s, the Republican said he would visit the state on Sunday, the day after his running mate, Rep. Paul Ryan (Wis.), is scheduled to stop in Harrisburg.

Romney and his allies have been running television ads in the state, and on Thursday an official at the Republican National Committee said the organization has bought $2.5 million worth of broadcast television advertising there, as well as about $500,000 in radio ads.

Both sides had considered the state out of reach for the Republican ticket, and Sen. John McCain (Ariz.) lost it in 2008 — after also visiting the Sunday before the election. But polls in the state have tightened recently, in part because the president has grown more unpopular in southwestern Pennsylvania, where he has been criticized as being a foe of the coal industry.

A Romney adviser cast the move as as one of strength. “Our campaign’s decision to travel to and campaign in Pennsylvania is just the latest example of how the race is breaking toward Governor Romney and allowing our campaign to be on offense,’’ said the adviser, who spoke on the condition of anonymity because the candidate’s travel plans have not been announced.

The Obama campaign has said Romney’s strategy — which also includes an effort in traditionally blue Minnesota — is born of desperation because his path to the 270 electoral votes he needs is blocked in key battleground states, including Ohio.

But even some Democrats in Pennsylvania have said Romney is smart to spend money in the state, given that he has cash to burn. And although there has been no indication that Obama will stump there, his campaign announced Thursday that Vice President Biden’s wife, Jill Biden, will visit Pennsylvania on Friday and Saturday.

In his tour of Virginia on Thursday, Romney returned to the core argument that has defined his candidacy: that he is a champion of business whose policies would usher in new jobs and rising incomes.

The former chief executive pounced on a proposal that Obama reintroduced this week to consolidate multiple government agencies under a single secretary of business, charging that it shows the president lacks the business know-how to jump-start the economy.

“I don’t think adding a new chair in his Cabinet will help add millions of jobs on Main Street,” Romney said in Roanoke on the factory floor of a window and door manufacturer. “We don’t need a secretary of business to understand business. We need a president who understands business, and I do.”

As if on cue, Starbucks chief executive Howard Schultz — who had spent much of the past year urging voters to withhold their political contributions from a system he blasted as poisonous — declared Thursday that Obama deserves a second term. Schultz, who supported Obama in 2008, said the president has demonstrated “significant leadership” on the economy and in world affairs.

Romney’s campaign announced endorsements from 13 business leaders, including the founders or co-founders of Home Depot, Charles Schwab and Netscape.

On a separate front, Romney released an unusual video attacking Obama for high gas prices — which will air on monitors above pumps at gas stations — and another video in which he blames Obama’s policies for the demise of a barbecue restaurant in Richmond.

In a tough new Spanish-language ad in Florida, the Republican candidate said Obama is supported by controversial Venezuelan President Hugo Chavez and the niece of longtime Cuban leader Fidel Castro. The Hispanic vote is critical in Florida and other states, and the ad brought a blast from the Obama campaign, which called it “bluster.’’

Markon reported from Washington. David Nakamura contributed to this report from Green Bay, Wis.; Las Vegas and Boulder, Colo.