Michael Barone applies his electoral number-crunching expertise to the case of white working-class voters and their importance to Mitt Romney’s presidential bid.

White working-class voters — or white voters without college degrees, the exit-poll group most closely approximating them — are now a mainstay of the Republican coalition.

Ronald Brownstein, a clear-sighted and diligent analyst of demographic voting data, provided some useful perspective in his most recent National Journal column. His bottom line is that in order to win this year, Mitt Romney must capture two-thirds of white non-college voters — about the same percentage that voted for Ronald Reagan in his 1984 landslide reelection.

The reason Romney must do so well is that white non-college voters are a smaller part of the electorate now than they were then. In 1984, they constituted 61 percent of all voters. In 2008, they constituted 39 percent.

The good news for Romney is that Republicans have been running near these levels for some time. In 2008, the white non-college vote went 58 to 40 percent for John McCain. In 2010, the white non-college vote for the House of Representatives was 63 to 33 percent Republican. Current polling shows Obama at about 33 percent among this group.