LAS VEGAS (AP) -- In a Las Vegas subdivision, Richard and Jessi Constantine recently found themselves alone, wheeling their 1-year-old son in a stroller as they knocked on doors to promote Mitt Romney's candidacy.

The Constantines were alone, politically speaking, but surrounded in the desert metropolis by volunteers working to re-elect President Barack Obama.

The Obama campaign's dominance of the ground game -- the volunteer-driven nuts and bolts of electioneering that ranges from registration drives to door-to-door canvassing -- contributed mightily to his 2008 victory.

Four years later the economy is floundering and the president's poll numbers are subpar, but his re-election campaign is banking on its advantage on the ground, assisted by a new array of digital innovations, to deliver victory once again.