CHARLOTTE, N.C. -- With his presidency on the line, President Barack Obama asked voters for more time Thursday, acknowledging that despite his lofty goals of hope and change, the economy will need years to recover.As he sought to regain trust from a disaffected electorate weary of months of high unemployment, Obama warned of tough times as the nation emerges from what he said are "challenges that have built up over decades."But he offered a rousing defense of his stewardship and insisted that his vision -- not Republican rival Mitt Romney's -- will lead to prosperity for the middle class."Know this, America: Our problems can be solved. Our challenges can be met," Obama said. "The path we offer may be harder, but it leads to a better place. And I'm asking you to choose that future."In a speech that at times seemed far from the soaring rhetoric of his first nomination address, Obama asked the crowd to "rally around a set of goals for your country."After attacking Romney for failing to offer specific proposals at the Republican National Convention, Obama outlined a series of goals including creating 1 million manufacturing jobs, recruiting 100,000 math and science teachers, and reducing the deficit by more than $4 trillion."They want your vote, but they don't want you to know their plan," Obama said. "And that's because all they have to offer is the same prescription they've had for the last 30 years."Republicans immediately called the goals scaled-down or recycled promises from 2008.With polls showing the race even, Obama needed to galvanize a dispirited base and persuade the last few undecided voters that his prescription for prosperity will restore the middle class.Obama cast the election as a choice between "two different paths for America," warning that "over the next few years, big decisions will be made in Washington, on jobs and the economy; taxes and deficits; energy and education; war and peace."Obama blamed Republicans for preventing him from accomplishing more, but he did not fully explain how he would work with them in the future.He took swipes at Romney, particularly on foreign policy, but he mentioned him by name only once, when he spoke about his disdain for providing tax breaks to the wealthy."No party has a monopoly on wisdom. No democracy works without compromise," Obama said. "But when Gov. Romney and his allies in Congress tell us we can somehow lower our deficit by spending trillions more on new tax breaks for the wealthy -- well, you do the math. I refuse to go along with that. And as long as I'm president, I never will."And he stressed that he's well aware of tough times ahead in fixing the economy. "I never said this journey would be easy, and I won't promise that now. Yes, our path is harder -- but it leads to a better place," Obama said.He delivered a forceful defense of his record but also asked for patience, invoking Franklin D. Roosevelt's efforts to bring the U.S. out of the Great Depression -- "the only crisis worse than this one.""I won't pretend the path I'm offering is quick or easy. I never have," Obama said. "You didn't elect me to tell you what you wanted to hear. You elected me to tell you the truth. And the truth is, it will take more than a few years for us to solve challenges that have built up over decades."And he made the case for a government role, saying the recovery "will require common effort, shared responsibility, and the kind of bold, persistent experimentation that Franklin Roosevelt pursued."Republicans have lambasted Democrats for looking to government to solve problems, and Obama included a caution to his own side, noting that it "should remember that not every problem can be remedied with another government program or dictate from Washington."He sought to use his platform to woo the last undecided voters. Campaign officials said Americans are just starting to pay attention after returning from their summer vacations and Labor Day holidays. As he arrived onstage, the crowd chanted "Four more years" and waved blue "Forward" signs.Hour after hour Thursday, delegates heard praise for Obama's accomplishments -- acting to stabilize the economy, passing healthcare legislation that Democrats have sought for decades, rescuing the auto industry and ending the war in Iraq.Obama's message, though, could be eclipsed today by the release of an August jobs report. A vast improvement isn't expected over the sluggish growth that has dogged his administration and kept the unemployment rate stuck at over 8 percent for 42 months of his term.Obama sought to excite his base hours before his address, apologizing via conference call to thousands of disappointed supporters who held tickets for a scrapped outdoor stadium address."I need you to remember that nothing is more powerful than the work that you guys do," he told supporters. "Nothing is more powerful than voices calling for change."Polls suggest that voters are uncertain about Obama's ability to return the U.S. economy to full health, and he promised a speech "laying out what are the stakes in this election and what my vision for the future is."Biden's speechDelegates streamed into the arena hours before Obama's arrival on the final day of the convention, which included a speech from Vice President Joe Biden, who has hit some rocky patches on the campaign trail. Biden's son, Delaware Attorney General Beau Biden, officially nominated his father for vice president.In his acceptance speech, Joe Biden relished the role of attack dog that vice presidents play, but he also sought to give an inside look at the steep challenges that Obama faced when he came to the White House."Folks, I've watched him," Biden said. "He never wavers. ... He asks the same thing over and over again: How is this going to work for ordinary families? ... And because of the decisions he's made, and the strength the American people have demonstrated every day, America has turned the corner."