Shocked by Nate Silver's GOP Senate prediction? Don't be

High profile prognosticator Nate Silver caused great consternation among Democrats with his recent forecast that the party is more likely than not to lose six seats and control of the Senate to Republicans in the 2014 midterm elections.

Democrats, and the public, should not have been surprised by the prediction.

Nate Silver
Jeremy Sutton-Hibbert | Getty Images

Others of us have been saying for months that the Senate would be at high-risk for Democrats who face major obstacles including an unpopular president, an unpopular health-care law and a sluggish economy.

There is also the tough historical trend in which the party controlling the White House generally loses seats in the sixth year of an incumbent president's term.

Since 1910, only Bill Clinton in 1998 bucked the sixth-year curse. The incumbent party's average loss of Senate seats since World War II? Six seats. Or exactly what the GOP needs.

On top of that, Democrats face a very difficult set of Senate races with far more vulnerable seats than Republicans.

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The GOP faces potential losses only in Kentucky and Georgia, and both are long shots for Democrats. Republicans are heavy favorites to pick up Democratic seats in Montana, South Dakota and West Virginia. The GOP is also favored to defeat Sens. Mary Landrieu in Louisiana and Mark Pryor in Arkansas.

That would leave Republicans in need of winning just one of the tossup races in Michigan, Alaska and North Carolina. The GOP also has outside shots in Colorado, Iowa and New Hampshire.

So the odds seem to favor the GOP hitting the number its needs to make Mitch McConnell of Kentucky the majority Leader.

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Some Democrats complained that Silver and FiveThirtyEight.com used partisan or out-of-date public polling to craft his forecast. That sounds disturbingly like complaints from Republicans in 2012 that forecasts of a big re-election win for President Barack Obama were based on "skewed" poll results.

Some Democrats also noted that Silver got predictions of some Senate Democratic races wrong in 2012. That's true, but the electorate in 2014—far less young and diverse—should favor the GOP much more than the 2012 race in which the president and his get-out-the-vote machine helped drive up numbers and improve demographics for Democrats up and down the ticket.

And beyond the individual states, the national mood heading into Election Day matters a great deal. While much can change, the problems for Democrats are stacking up.

The most recent NBC/Wall Street Journal poll showed Obama with a record low approval rating of just 41 percent and 54 percent disapproving of the job he is doing. The Affordable Care Act, which Republicans plan to hammer all year, was viewed as a "bad idea" by 49 percent and a "good idea" by just 35 percent.

Democrats do have a slight edge in most "generic ballot" surveys. But as Silver pointed out, these polls tend to oversample Democrats not likely to show up in the midterms.

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On the economy, one might think Democrats would have a slight advantage given that the unemployment rate is dropping and most indicators suggest at least modest growth for the rest of the year.

But what matters in voting is not raw numbers but people's perceptions of the economy. And on that score Democrats are in serious trouble.

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The latest Gallup Economic Confidence Index showed that just 19 percent of Americans "view the economy as excellent or good and 37% say poor." Meanwhile, 39 percent say the economy is getting better and 56 percent say it is getting worse.

That kind of economic pessimism, coupled with history, an unpopular incumbent, a tough issue environment and a brutal set of battlegrounds should strike fear in every Democrat's heart.

—By Ben White. White is POLITICO's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet POLITICO Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.