I could argue Obamacare either way so I'll call it a wash. To me it boils down to:
1) is there any positive change in the economy? If not; advantage Romney
2) does Obama have a strategy - so far his message is "more of the same" and "the other guy is dangerous". Coupled with #1 if he doesn't put forward some plausible vision; advantage Romney
3) will Republicans muck things up by 1) throwing social issues in, 2) over pressing the Walker win. If so; advantage Obama.
4) will opposition research and/or the press really hammer Romney? If so; advantage Obama.
As long as Romney looks like a safe choice to Independents I can see him winning and possibly winning by 3% or more. The concern is that blue states may turn purply-blue and red states get really red but in the end there are more blue than red (even though Romney has more votes).