By Ashley Killough and Tom Cohen, CNN
CHARLOTTE, North Carolina (CNN) - Facing a close election and Republican attacks that they have made things worse while in power, President Barack Obama and Democrats seek to emphasize what has been achieved and additional steps to bolster the middle class at their three-day national convention that begins Tuesday.
The political conclave that will formally nominate Obama for a second term serves as a response to last week's Republican convention that nominated Mitt Romney as the GOP challenger in November.
Democrats offered a glimpse of issues expected to play a prominent role in this week's events, releasing their party platform late Monday. It focuses on improving the economic situation for middle-class Americans, a central theme of Obama's campaign and an issue the party hopes will win votes come November. It also contains language endorsing same-sex marriage for the first time, a move that brings the party's official stance in line with that of the president, who said for the first time in May that he supports marriage rights for gays and lesbians.
"Over the course of the week, you'll hear a very different tone than the one that you heard last week in Tampa, which was really essentially one nonstop series of attacks on President Obama," Rep. Debbie Wasserman Schultz, the chairwoman of the Democratic National Committee, told CNN on Tuesday.
"We're going to lay out the case for moving the economy forward. President Obama and speakers throughout the week will talk about and have an honest conversation about where we were when he first took office and where we are now after four years of his policies and 29 straight months of job growth in the private sector. And that we need to continue to move forward and we've got a ways to go."
First lady Michelle Obama will address the convention Tuesday night, and former President Bill Clinton headlines the second night before Obama concludes it with his nationally televised address Thursday night.
Romney's campaign is focused on the question of whether Obama has made life better for Americans, arguing that continued high unemployment and a sluggish economic recovery from the 2008 recession show that White House policies have failed to deliver promised results.
The Democratic platform points to accomplishments made over the past four years, but argues for a second Obama term by saying more work remains on fixing the economy and making the country more secure. It also pins the blame for high debts and a diminished jobs outlook on former President George W. Bush's administration.
The "are you better off" strategy was famously employed in 1980 by Ronald Reagan, who asked voters that question when running against incumbent Democrat Jimmy Carter at a time of national economic woes. Reagan went on to win, and the Romney campaign has repeatedly invoked his name this year while seeking to link Obama and Carter as failed leaders.
Obama and Vice President Joe Biden launched their counterattacks Monday, telling supporters at Labor Day campaign events that the nation is better off now than it was when the new administration took office in January 2009.
Noting that Romney recently said the nation needed a "new coach," Obama told a Toledo, Ohio, event that the former Massachusetts governor offered nothing new from Republican policies that Obama said failed in the past.
"The problem is everybody's already seen his economic playbook. We know what's in it," the president said.
Biden led an effort to sharpen the message of Democrats after other senior party members struggled to formulate a definitive answer to the question of whether voters should feel better off since Obama took office.
"America is better off today than they left us when they left," Biden told a Labor Day campaign event in Detroit, referring to the state of the nation the Obama administration inherited from the Bush White House.
"You want to know whether we are better off?" Biden asked, offering a favorite campaign line. "I've got a 'better off' -- Osama bin Laden is dead and General Motors is alive!"
Romney's running mate, U.S. Rep. Paul Ryan, kept up the Obama-Carter comparisons, telling a North Carolina crowd that "every president since the Great Depression who asked Americans to send them into a second term could say that you are better off today than you were four years ago, except for Jimmy Carter and for President Barack Obama."
The back-and-forth between the campaigns is part of their competition on how the election gets framed in the minds of voters. Republicans want it to be a referendum on Obama's presidency, while Democrats seek a choice between differing political ideologies on the size and role of government.
In particular, Republicans seek to shrink the size of government and end chronic federal deficits and rising national debt through reduced spending, reforming entitlement programs such as Social Security, Medicare and Medicaid, and slashing taxes on businesses and individuals as a spur for economic growth.
Obama and Democrats argue that a deficit-reduction plan also needs additional revenue as part of the equation, and they propose allowing tax rates on income of more than $250,000 for families and $200,000 for individuals to return to higher levels from the 1990s.
Republicans oppose any kind of tax increase, and the impasse over that issue has been the main impediment to a comprehensive deficit-reduction agreement during Obama's first term.
A senior Obama campaign official told CNN on Monday that viewers tuning in to the Democratic convention will hear how specific Obama policies and ideas will bolster the middle class to strengthen economic growth.
"You didn't hear one tangible idea" at the GOP gathering in Tampa, Florida, the official argued, adding that "the advantage of going second is you get the last word."
The race overall is very tight, with a poll Monday indicating Romney holds a slight advantage in North Carolina. Obama narrowly won the battleground state four years ago, becoming the first Democrat to carry it in a presidential election since Carter in 1976.
"Your country needs your help," Ryan told an overflow crowd at East Carolina University in Greenville, in the eastern part of the state.
Meanwhile, Ryan continued to be hounded by false statements in his convention speech last week, coming out again Tuesday to defend a line in his speech that appeared to blame Obama for the closure of a General Motors plant in Janesville, Wisconsin. An announcement that the plant would close preceded Obama's election.
"What I was saying is the president ought to be held to account for broken promises," Ryan said on NBC. "After the plant was shut down, he said he would lead efforts to restart the plant. It's still idle."
Ryan also addressed his erroneous claim to have run a three-hour marathon in the past.
"I thought I ran an ordinary, kind of normal time, and I thought that was an ordinary time, until my brother showed me a three-hour marathon is crazy fast," Ryan responded when asked about the marathon claim in an interview with WTOL. "I ran a four-hour marathon. And so it's just the fact that I did this 22 years ago, and I forgot what my time was and that's what I thought it was."
CNN's Adam Aigner-Treworgy, Kevin Liptak, Peter Hamby and Paul Steinhauser contributed to this report.