Having helped engineer two electoral victories for George W. Bush, Karl Rove‘s take on electoral math is worth noting. He explains for Wall Street Journal readers why he believes the numbers look good for Republican challenger Mitt Romney.

He maintains a small but persistent polling edge. As of [Tuesday] afternoon, there had been 31 national surveys in the previous seven days. Mr. Romney led in 19, President Obama in seven, and five were tied. Mr. Romney averaged 48.4%; Mr. Obama, 47.2%. The GOP challenger was at or above 50% in 10 polls, Mr. Obama in none.

The number that may matter the most is Mr. Obama’s 47.2% share. As the incumbent, he’s likely to find that number going into Election Day is a percentage point or so below what he gets.

For example, in 2004 President George W. Bush had 49% in the final Gallup likely-voter track; he received 50.7% on Election Day. In 1996, President Clinton was at 48% in the last Gallup; he got 49.2% at the polls. And in 1992, President George H.W. Bush was at 37% in the closing Gallup; he collected 37.5% in the balloting.

One potentially dispositive question is what mix of Republicans and Democrats will show up this election. On Friday last week, Gallup hinted at the partisan makeup of the 2012 electorate with a small chart buried at the end of its daily tracking report. Based on all its October polling, Gallup suggested that this year’s turnout might be 36% Republican to 35% Democratic, compared with 39% Democratic and 29% Republican in 2008, and 39% Republican and 37% Democratic in 2004. If accurate, this would be real trouble for Mr. Obama, since Mr. Romney has consistently led among independents in most October surveys.

Gallup delivered some additional bad news to Mr. Obama on early voting. Through Sunday, 15% of those surveyed said they had already cast a ballot either in person or absentee. They broke for Mr. Romney, 52% to 46%. The 63% who said they planned to vote on Election Day similarly supported Mr. Romney, 51% to 45%.