Obama likely needs more than 39 percent of whites to assure re-election. Romney likely needs at least 61 percent of whites to assure Obama’s defeat (or 60.5 in some scenerios). These are estimates based on an electorate that matches the diversity of 2008 or is slightly less white. It presumes the Electoral College outcome does not diverge from the winner of the popular vote (loose talk aside, it’s only happened four times in U.S. history)."[Obama's White Support Is Too Low to Win, By David Paul Kuhn, June 22, 2012]
The "white margin to watch"... is that the way we talk now? I find that pretty unpleasant, and it's also odd to be talking with such specificity — 61-39! — while rejecting the powerfully specific mechanism that is the Electoral College. Once you tip to a plurality in a state, it doesn't matter how many white or nonwhite voters go this way or that (except in Maine and Nebraska). So polling that ignores the Electoral College is inherently inaccurate in a close election.
One reason it's "the way we talk now" is that Obama gets 95 percent or more of the black vote, and 69 percent of Hispanics, so the white vote is the only one that's in play.
The other reason is that it's the way professional politicians have always talked, although being professional politicians, they haven't always talked that way in public.
"Owing in part to Article V of the Constitution, it is impossible to amend the provision that grants two senators to each state of the Union, regardless of population. Thus - in an arrangement aptly described by Daniel Lazare as one of "rotten boroughs" - unpopulous white and rural states such as Montana and Wyoming have the same representations as do vast and all-American and ethnically diverse state like New York and California."