One of the beneficiaries of the new GOP map, Rep. Steve Chabot, won a district in 2010, where Sen. John McCain (R-Ariz.) had taken just 45 percent of the vote in the 2008 presidential election. Chabot’s new district, though, would have given McCain about 52 percent. (Al Behrman/AP)

The fight for the GOP presidential nomination will hit a fever pitch on Super Tuesday but the pivotal election day also will mark the official beginning of the battle for control of Congress.

Ohio will hold the nation’s first congressional primaries Tuesday, giving Republicans a chance to begin reaping the benefits of their big wins in the 2010 midterm elections. Because of those victories, Republicans control both the legislature and the governor’s mansion in so many states that they were able to redraw four times as many congressional districts as Democrats.

The question they face now is whether they can turn those advantages into seats.

A review of the 66 districts that flipped from Democratic control to GOP control after the 2010 elections shows that Republicans have emerged with a better map than they had before redistricting, but in many cases, those improvements were slight. Most Republicans who were vulnerable before will continue to be at risk over the next decade.

Despite the GOP’s unprecedented control over the drawing of new lines, there is little room to grow. The party’s big majority makes creating new seats to win much more difficult. There are only so many ways to redraw a map, and Republicans had more incumbents to protect.

The Washington Post has projected that both Republicans and Democrats have come away with about the same number of new districts that they should be able to capture — a result that has led some to declare the 2012 redistricting process a wash.

But that’s only one part of the equation.

The other part is where the GOP’s real advantage lies: in improving on the districts it already controls, or “shoring up” its incumbents.

“They’ve done a great job,” said former congressman Thomas M. Davis III (Va.), who oversaw the GOP’s last redistricting cycle, in 2002, as chairman of the National Republican Congressional Committee.

Davis said that even the slightest changes can be a big help: “You drive off a lot of opposition. Then after that, on election day, maybe you win by a few thousands votes.”

Minor differences

A review shows that many of the changes made for this year’s vote have indeed been minor.

Of the 66 districts that switched from Democratic to Republican after 2010, Republicans were able to redraw 37, while Democrats got to redraw nine.

Of the 56 districts that have been redrawn, more than half (32) became more Republican-friendly, 14 districts got more difficult for Republicans, and the remaining 10 districts basically will stay the same.

A further breakdown shows that although most of the districts will become better for the GOP, the difference is often pretty minor. Of the 32 districts that become more Republican, about half (15) will become only 1 percent or 2 percent more so, and only 10 will become at least 5 percent more so.

Similarly, more than half of the districts that become more Democratic will be only 1 percent or 2 percent bluer.

In the end, 35 of the 56 districts that have been redrawn will change by 2 percent or less — a strong sign that these Republicans will continue to have to watch their backs in November, even if most of them are safer.

“If you sum all this up, I think it supports the case that the Republican plan to shore up freshmen reaped only minor gains, and may have been too conservative, given their control of the process,” said Mark Gersh, a Democratic redistricting expert.

Ohio is a good example of Republicans’ more successful efforts to bolster their incumbents.

Republicans already control 13 of 18 congressional seats in this crucial presidential state, which lost two spots as a result of population shifts. So they had to pack as many Democrats into as few districts as possible to protect their members.

In the end, they created four Democratic districts and did their best to protect the 12 remaining GOP-held districts.

The result was that each party is likely to lose one seat.

And although on the surface that looks like a wash, Republicans were able to use their advantage to make three of their most vulnerable members much safer. Not so for Democratic Reps. Marcy Kaptur and Dennis J. Kucinich, whose districts were merged into one. The two will face off Tuesday in the marquee congressional primary.

Beneficiaries of new lines

Two of the beneficiaries of the new GOP map — freshman Reps. Steve Stivers and Steve Chabot — each won districts in 2010, where Sen. John McCain (R-Ariz.) had taken just 45 percent of the vote in the 2008 presidential election. Chabot’s new district, though, would have given McCain about 52 percent, while Stivers’s would have given him 53 percent.

The changes should help Stivers and Chabot win in districts that otherwise would have been top Democratic targets.

“This is my first reelection campaign and I am taking this race as seriously as I did in 2008 and 2010, when the district was one of the most competitive in the nation,” Stivers said.

Despite his careful approach, Stivers got one of the biggest bumps in the country as a result of redistricting. Compared with the 65 other Republican freshmen who won Democratic seats in 2010, he had the fifth-biggest overall improvement.

These freshmen represent most — though hardly all — of the most vulnerable Republicans in Congress. But not all of them are considered top targets, as some won conservative districts that will be difficult for Democrats to take back.

Some of the districts are also in limbo, with a court challenge potentially affecting a few Republican freshmen in Florida and Texas and an important map in New York that will be drawn by a split legislature or the courts.

Regardless of what happens, though, the overall picture is starting to come into focus: The map that Republicans have to protect their majority in 2012 will be more GOP-friendly — although not hugely so — than the map with which they won the majority in 2010.