The Obama 2012 presidential campaign, which has now officially sprung to life, confronts a vexing political puzzle. The unemployment rate is plummeting. After the March jobs report release, White House economic adviser Austan Goolsbee pointedly noted that the full percentage-point decline over the past four months is the largest such drop since 1984.

That statistical coincidence dovetails neatly with this David Axelrod-endorsed narrative: Just as Ronald Reagan bounced back from a nasty first-term recession to win re-election in 1984, a jobs rebound will mean four more years for Barack Obama. Got that, MSM? Obama 2012 = Reagan 1984. Now shut your laptops and run along.

But as the Obama political shop has surely noticed, the unemployment rate isn’t the only politically important number on the decline. Simultaneously, their boss’s approval rating has fallen from 51.0 percent on Jan. 24 to 47.4 percent today, according to the RealClearPolitics poll average. A large-sample Quinnipiac survey out last week had Obama at 42 percent. And a recent Reuters-Ipsos poll found that Americans’ confidence in the way the country is going has slumped to its lowest point of Obama’s presidency with 64 percent believing the nation is on the wrong track. Even as more jobs are being created, so are doubts about Obama.

Keep in mind that forecasting models suggest a president with a 50 percent approval rating on Election Day has an 80 percent chance at re-election vs. just a one-in-three chance for an incumbent with a 45 percent rating. And polling analyst Nate Silver notes that every incumbent with an approval rating of 49 percent or higher since World War Two won re-election, while every candidate with a rating of 48 percent or lower lost.

Morning in America 2.0, Mr. Axelrod? More like Threat Level: Midnight. And here’s why: While jobs are growing, incomes are not. And income growth — or the lack of it — political scientists agree, is the economic variable with the most impact on national elections. Strong growth in real disposable personal income led to huge victories for Reagan in 1984, Richard Nixon in 1972 and Lyndon Johnson in 1964. Weak or negative growth doomed Jimmy Carter in 1980, George Bush in 1992 and John McCain in 2008.

Real disposable personal income fell 0.1 percent in February. Average hourly wages were flat in March, and have grown at a 1.8 percent annualized rate over the past three months, according to the Economic Policy Institute. With inflation running around 2 percent, this means the average American is falling behind, his standard of living dropping. As the Brooking Institution figures things, between October 2010 and February 2011, real hourly and weekly earnings in the private sector fell 1.1 percent.

Even Goolsbee knows those numbers won’t improve a whole lot unless the unemployment rate moves sharply lower. Yet the official White House economic forecast has unemployment averaging 8.6 percent in 2012, not much below the current 8.8 percent rate. (The broader U-6 rate, which includes discouraged workers and part-timers who want full-time gigs, is a sickening 15.7 percent.) JPMorgan economist Michael Feroli thinks a combination of so-so economic growth, a vast pool of unemployed, higher energy prices and the expiration of the 2011 payroll tax cuts means income growth will likely remain “tepid” going forward.

So for now, consider Obama a favorite to win a second term — most presidential incumbents do — but only by the narrowest of margins. If incomes stay stagnant — and if Republicans can nominate someone with a strong, passionate and specific pro-growth economic message — Election Night 2012 could be a long one.