Over the last several weeks, President Obama has widened his polling lead over Mitt Romney in several key battleground states, surging for example in in once-waffling Michigan and Pennsylvania, which now appear to be out of reach for the GOP. As a result, even though the rival candidates are neck-and-neck in most national polls, Romney's path to electoral victory is getting narrower. According to Real Clear Politics' polling map, Obama has a clear advantage in 20 states with a total of 247 Electoral College votes. If he wins those, he'll only need one or two of the eight true toss-up states — Colorado, Florida, Iowa, North Carolina, Virginia, Ohio, Nevada, and New Hampshire — to get to the 270 votes needed to win a second term. Of course, with six weeks till Election Day, Romney still has time to turn things around, especially with three debates still to come, says Edward J. Rollins in the New York Daily News. Romney can win over doubters by appearing "presidential, calm and knowledgeable." He also has to stamp out infighting in his campaign, stay on message, and focus on must-win states. And something big, such as a new economic crisis or trouble in the Middle East, could still trip up Obama. But in the end, winning the presidency comes down to electoral math, and at the moment New York Times statistical guru Nate Silver gives Obama roughly a 75 percent chance of victory. What are the equations that would put Romney in the White House? Here are three:

1. Follow George W. Bush's roadmap
One obvious path for Romney would be following Bush's path to victory in 2004, says Chris Cillizza at The Washington Post. "At the moment that looks like a dicey proposition, and that should make Republicans very nervous." Still, it's worth a look. If Romney won every state Bush did, he'd have 292 electoral votes, so he's got a little room for error. Bush's tally included six wins in what were then considered swing states — Nevada, Colorado, New Mexico, Iowa, Ohio, and Florida. New Mexico and Iowa will likely go to Obama, so subtract their electoral votes, and Romney is down to 281, barely enough to win. And remember, there are other obstacles on this path, too, as "some states that President Bush never worried about — Virginia and North Carolina in particular — are quite competitive" this year.

2. Start with McCain's states, and build from there
"The first thing Romney must do to win is to hold on to all of the states that Sen. John McCain (R-AZ) won in 2008," says Ryan Witt at Examiner.com. The good news for Romney is that this "crucial first" appears a safe bet. But that only gives him 180 of the 270 electoral votes he needs. Romney's next step is harder. He'll need to pick up Indiana and North Carolina, two states Obama won by less than 1 percent in 2008. Assuming he can pull that off, Romney will be up to 206 electoral votes. Then the going really gets tough, as he'll have to get 64 more electoral votes from the remaining toss-ups. He really must win Florida, because without its 29 electoral votes, Romney would essentially have to win all the other swing states. And even with Florida in the GOP column, without Ohio, where Obama now leads, "the math is nearly impossible."

3. Flip states where he's down but not out
Can Romney still win if he loses Ohio and Virginia, two states once viewed as must-wins for the GOP but where Obama has pulled into a solid lead? "Well, it's possible," says Mark Halperin of TIME. If you accept that Michigan and Pennsylvania, two GOP dream pick-ups, have definitively shifted to Obama, there's still a way for Romney "to get to exactly 270 without Ohio and Virginia." It involves flipping "five states where he's currently down: New Hampshire, Wisconsin, Iowa, Nevada, Colorado." Of course, it's a given that for this formula to work Romney has to take Florida's 29 electoral votes in "a state Obama (not McCain) won in 2008." That's "no sure thing for the Republicans this year by any means."

Read more political coverage at The Week's 2012 Election Center.