(Getty Images/Marty Melville)

(Bloomberg) – Some political analysts say the Republicans have a shot at gaining the six seats they need to control the Senate in the upcoming November elections.

“The Republicans are at least even money — and maybe a little better than that — at taking over the Senate,” according to Stu Rothenberg, editor of the nonpartisan Rothenberg Political Report.

“It’s too early to make a precise prediction, except to say that Democrats are nearly certain to lose Senate seats,” Sam Wang, who since 2004 has used mathematical formulas and polling data to predict elections for the Princeton Election Consortium, said in an e-mail.

Democratic efforts to maintain control are burdened by relatively low approval ratings for President Obama and the Patient Protection and Affordable Care Act (PPACA).

The Democrats are defending more seats than the Republicans are, and the Republicans could benefit from midterm electorate demographics: Midterm voters tend to be older, whiter and more Republican than the voters who show up for presidential elections.

Since the end of World War II, the party allied with the president has lost ground in the Senate in 12 of 17 midterm elections.

Wang, a neuroscientist at Princeton University, said major factors that will go into his calculations include Obama’s approval ratings and whether Republicans can field candidates that will appeal to a broader electorate.

In recent elections, Republicans have nominated Senate candidates who “appealed to primary voters, but who alienated voters in the general election,” Wang said.

Among the examples he cited was when then-U.S. Rep. Mike Castle of Delaware was upset in a 2010 Republican Senate primary by Christine O’Donnell, a favorite of the limited-government Tea Party movement who during the general election campaign was forced to deny she was a witch and went on to lose to a Democrat.

“In other words, Republicans run the risk of underperforming expectations,” Wang said. “Basically, the odds are 50-50, and anyone who makes a more precise prediction is out on a limb.”

Jennifer Duffy, who studies Senate races as a senior editor for the nonpartisan Cook Political Report in Washington, agreed with that analysis.

“I think we stand at a jump ball,” she said. “We need some primaries to get out of the way, from Georgia to North Carolina, before we’ll know more.”

Obama would have a weaker negotiating position if Republicans held both the House and Senate, and that would also likely make it harder for him to get his judicial and other appointees confirmed. He would also likely spend some of the last two years in office vetoing anti-Obamacare legislation.

“It would cripple the presidency,” Rothenberg said.

As recently as October, Democrats held a six percentage point advantage in which party voters prefer to control Congress, according to an aggregate of polls by the Real Clear Politics website.

After the glitch-plagued rollout of the PPACA exchange system, the two parties are now roughly even on that question.

The most recent three-day average for the president in Gallup’s tracking poll shows him with 51 percent of Americans disapproving of the job he’s doing and 44 percent approving.

A bigger threat to Democratic chances than the president’s poll ratings is the playing field. The party has greater exposure to potential losses because they’re defending 21 Senate seats, compared to 15 for Republicans.

“You could almost predict the day after the 2008 elections that the Democrats would be likely to lose some seats in the Senate six years later, just like the Republicans can be expected to lose some seats in 2016 because they won so many seats in 2010,” said Alan Abramowitz, a political scientist at Emory University in Atlanta.

“The Democrats who are retiring, plus the ones who are likely to lose, they tend to be more moderate Democrats,” Abramowitz said, adding that any Republicans who will replace those Democrats “are going to be very conservative.”

Abramowitz has a statistical model that shows the Democrats could lose five or six seats.

The variables in the model include control of the White House, the difference between the number of Democratic-held seats and Republican-held seats at stake in the election, and how the two national parties perform on a generic congressional ballot test shortly before the election.

“It will change depending on what happens to that generic ballot question,” Abramowitz said. “That’s the only predictor that’s really not fixed at this point.”

Some of the Democratic Senate incumbents rated as most vulnerable include John Walsh in Montana, Mark Pryor in Arkansas, Mark Begich in Alaska, Mary Landrieu in Louisiana and Kay Hagan in North Carolina.

The only apparent opportunities for Democratic pickups are in Kentucky, where Minority Leader Mitch McConnell has a primary challenger and a potentially strong Democratic opponent, and in Georgia, where Democrats are banking on a fractious Republican primary that includes three House members.

Some say Democrats could have a chance at competing in Republican-leaning Mississippi if six-term Sen. Thad Cochran loses the Republican primary to state Sen. Chris McDaniel, who’s aligned with the Tea Party movement.

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