Romney's situation is almost the mirror opposite. Because voters have already decided they disapprove of Obama's handling of the economy, Romney doesn't want to change voters' minds about Obama--and he probably couldn't if he tried. And since Obama is already president, the baseline questions of what kind of president he would be are moot, leaving Romney little incentive to attack Obama's character. Voters are unlikely to shift their opinions about Obama too much in either direction. They have already decided they want change; they just need a credible alternative.
If Romney's task is to present himself as a credible alternative to Obama, how should he go about doing it? First, Romney needs to avoid pitfalls and unforced errors that would disqualify him. Second, Romney needs a positive campaign about himself.
Every campaign involves both positive ads and negative ads. Negative ads are the mudslinging attack ads that can become famous or infamous each election year. Positive ads are the "feel good" ads that a candidate runs about himself to advertise his credentials and positive qualities. It should be no surprise that Obama will focus on creating negative ads about Romney. Romney will to use negative ads too, but he'll also need to put a focus on positive ads.
At present, most voters know little about Romney. His national profile consists of his 2008 and 2012 runs in the Republican presidential primary. But not too many voters actually pay much attention to primary campaigns. They'll note the results of the contests, perhaps, but unless they're in the political party of the primary and their state is hotly contested, they won't see much of the primary campaign itself. For example, only a small percentage of the general election voters casting ballots this November will have watched Romney's performances in the Republican primary debates.
If voters don't know much about Romney, how can they see him as a credible alternative to Obama? They can't. That's what positive ads are for. Note that the big-spending SuperPACs tend to run negative ads rather than positive ones. This means Romney's campaign itself will need to flood the airwaves with positive ads; they can't rely on outside sources. So far they haven't done this. By contrast, both the Obama campaign and his SuperPACs will be able to make an impact with negative ads against Romney. This disparity may be one of the reasons Romney has not achieved a real breakthrough in the national polls thus far.
But there's some good news for Romney. Voters begin paying close attention to the general election campaign beginning with the parties' national conventions, which will start in late August. The Republican national convention will be, in essence, a very big, expensive, highly-watched positive ad for Mitt Romney (and his VP nominee). At the national convention, the opponent will come under some attack, but the conventions tend to be positive. They tell the life story of the candidate and showcase the reasons why he should be president.
Thus, only after the Republican convention will we be able to say that most voters are familiar with Romney. That's when poll numbers will become more telling. In addition, the presidential debates held in October have the potential to serve as positive ads of a sort for the challenger. (If a candidate does poorly, they become like very effective negative ads.) If the challenger is put on the same stage as the incumbent president and he does well, the challenger is basically given free, positive advertising that says "this man is a credible alternative."
It's ironic that an incumbent president would need to go on the offensive, while the challenger is on the defensive in the sense that he needs to build himself up. This isn't always the case, but when the economy is in unusually bad shape and voters want change, that's the position they're in. With the convention and the debates, Romney has two big opportunities to make gains. Since Obama lacks equivalent opportunities, Romney is in better shape than the polls will suggest--until the conventions.