For President Obama, the fall campaign to hold the White House will begin this week with the Democratic convention in Charlotte, N.C., where the faithful hope to rev up the vast apparatus that produced his historic 2008 election.

Facing an energetic challenge from the freshly nominated GOP ticket of Mitt Romney and Paul Ryan, Obama advisers say they will use their convention to mobilize an army of volunteers that could help overcome the Republicans' anticipated cash advantage in the new era of unrestricted, big money electioneering.

"This is a chance to have a conversation with millions of voters, many of them still undecided," said Minnesota DFLer Jeff Blodgett, the Obama campaign's state director. "But because of the organization we have going on, we're able to also use this opportunity to take a big jump forward in terms of getting people involved."

As they fire up their ground game, Democrats also must counterpunch the Republican National Convention in Tampa, where Romney made an explicit appeal to voters disillusioned with Obama's promises of hope and change from four years ago.

The Obama campaign template -- modeled in no small part after the grass-roots campaigns of the late Minnesota Sen. Paul Wellstone -- could be a departure from the usual focus on glossy television imagery and parties hosted by lobbyists and special interest groups.

Obama partisans around the country, working off a list of 13 million e-mail addresses from the 2008 campaign, will host nationwide "watch parties" to draw in new supporters and volunteers. Blodgett said more than 100 are planned in Minnesota.

As in 2008, the highlight of the convention, which kicks off Tuesday, will be Obama's acceptance speech in an 80,000-seat stadium open to people outside the usual closed loop of delegates and party activists.

Organizers say there will be no shortage of made-for-TV imagery in Charlotte. Former President Bill Clinton will rally the crowds. But in a first, this convention is being mounted without direct contributions from political action committees, lobbyists and corporations. That self-imposed restriction, even with substantial loopholes, hindered fundraising and resulted in a decision to cut the festivities short by one day.

There also will be Obama's signature oratory, which lifted the party's spirits four years ago and helped elect the nation's first multi-racial president.

But all this, organizers say, will be harnessed to a central organizing principle: Energizing the grass-roots networks of volunteers in Minnesota and across the country who could provide the crucial difference in an election that may be decided by a sliver of independent voters in a half-dozen states.

Organizing opportunity

"For us in Minnesota," Blodgett said, "we see ... the night of the president's speech, to be a huge organizing opportunity."

It also will be a chance for Obama to counter the Republican narrative of failed promises, an 8.3 percent unemployment rate and a president who has created too few jobs for the struggling middle class.

Republicans spent last week wooing independents and moderates by saying that if it was understandable to give a fresh-faced senator from Illinois a chance four years ago, Obama's time is up and the nation is due for a change.

"You know there's something wrong with the job he's done as president when the best feeling you had was the day you voted for him," Romney told cheering Republicans in Tampa.

But Democrats headed to Charlotte this weekend believe they have a far different story to tell of the president's time in office -- bringing health care to the uninsured, ending the war in Iraq, reining in Wall Street, killing Osama bin Laden and averting an economic depression.

Minneapolis Mayor R.T. Rybak, an Obama supporter with a speaking role at the Democratic convention, said there is nothing faded about Obama's fans.

"Go to headquarters in Hopkins [Minn.] or in Durham [N.C.] and you will see people who are not only willing to vote for him, but to give massive amounts of time to help him win," Rybak said from North Carolina, where he is continuing his role as a Democratic National Committee vice chair. "That's pretty remarkable, given that he's had to navigate us through a massive crisis."

Instead of letting Obama volunteers go dormant after the last election, Democrats have given them better training and more recruiting tools to boost their chance of success.

A call to action

"These conventions are great, but the reality is we have an election to win here in Minnesota, and my hope is that this will be a great call to action," said DFL Party chair Ken Martin.

Democratic strategists are counting on a vigorous ground game to offset the advantage better-funded Republicans may have in the blitzkrieg of political ads that will saturate television next month.

According to August fundraising reports, Romney had $186 million cash on hand, a more than $60 million advantage over Obama's $124 million.

Locked in a statistical tie in national polls before the two conventions, the two campaigns are also using their money differently. While both campaigns have begun unleashing television ads, the president is ahead in the race to set up state field operations.

For example, Obama already has campaign offices across Minnesota. Romney, who has largely ignored the state except for fundraisers, is just starting to establish a beachhead here.

But Republicans and their allies are relying on more than ads. They'll also tap a growing grass-roots network of Tea Party groups, church-affiliated religious conservatives and perhaps even libertarian devotees of Texas Rep. Ron Paul.

In the run-up to the convention, Kimberly Howard, of St. Paul, hopes the hours she spent volunteering at Obama's St. Paul office will help make a difference.

Inspired to get involved in part after Wellstone died in a 2002 plane crash, Howard said she "truly felt the need to get involved and share my story."

That led to dozens of phone calls the week before the convention, as she dialed to reach "persuadables" -- voters who may not be with the president but could move his way.

"What has been just eye-popping for me is the thousands and thousands upon thousands of volunteers who are engaged in this campaign," said Obama's national political director, Katherine Archuleta, who travels the country speaking to activists. In 2008, it "was a campaign of change and this campaign is a campaign of urgency."

Rachel E. Stassen-Berger Twitter: @rachelsb Kevin Diaz is a correspondent in the Star Tribune's Washington bureau.