Democrats and the Senate

As the presidential primary season moves into high gear in Iowa this week, voters can be forgiven if they've failed to focus on what could be a developing tidal wave for the other elected branch.

Simply put, in a time of lingering economic malaise when incumbents of either party are none too popular, the road to maintaining the Democratic party's slim, 51-47 majority in the U.S. Senate is looking downright rocky.

The first problem is mathematical. Six-year senate terms mean only 33 seats in the senior house are up for grabs in 2012. It's the luck of the draw that 23 of those are currently held by Democrats.

But the bad news continues: Democratic Sen. Ben Nelson of Nebraska announced last week he won't seek re-election next year -- the seventh Democratic incumbent to throw in the towel.

A former two-term governor, Sen. Nelson was the Democrat best able to defend the seat in Republican Nebraska, Jennifer Duffy, senior editor of the non-partisan Cook Political Report, told Bloomberg News. Without him, "I don't see a path to victory for Democrats in Nebraska," she said.

On the other hand, Politico reports Sen. Nelson was also "considered one of the most endangered Democratic incumbents this cycle" -- he was actually booed out of a Nebraska pizza parlor after he was characterized as selling his vote to put Obamacare over a vital 2009 procedural hurdle in what became known as "the Cornhusker Kickback."

Meantime, only two Republican senators, Kay Bailey Hutchison of Texas and Jon Kyl of Arizona, plan to retire next year.

Interestingly, that leaves Nevada as one of the few opportunities for a Democratic gain, since it's "a swing state" where "the presidential race is going to have an impact over the outcome," according to Ms. Duffy. Appointed GOP Sen. Dean Heller faces a strong challenge from Democratic Rep. Shelley Berkley.

While the Democrats' best hope for picking up a GOP-held seat is still in Massachusetts, where Republican Sen. Scott Brown won a special election two years ago to replace the late Democratic Sen. Ted Kennedy, but will now be campaigning for his first full term.