To appreciate how the GOP’s new legislative redistricting plan protects the party’s incumbents, take a look at three state Senate districts in southeastern Wisconsin that have a recent history of competitive elections.

These are the seats now held by Alberta Darling of River Hills, Leah Vukmir of Wauwatosa and Van Wanggaard of Racine.

All three districts voted for Democrat Barack Obama for president in 2008 (by between four and 12 points).

All three voted for Republican Scott Walker for governor in 2010 (by between eight and nine points).

All three have seen tough, tight, expensive battles for state Senate over the past decade.

And all three would become far safer Republican seats under the GOP plan, shielding them from any serious Democratic challenge in the coming years, and giving Democrats fewer paths to winning the state Senate back over the next decade. In essence, the plan shifts each district westward to shed urban Democratic voters and gain suburban Republican voters.

This is classic partisan line-drawing. Republicans have undivided power in Madison. They can draw any map they want that meets constitutional standards. In this case, they are relieving Darling, Vukmir and Wanggaard of their least supportive constituents and depriving the other party of a base from which to challenge them.

As the saying goes, the politicians are picking their voters, rather than the other way around. In the process, the new plan greatly reduces the potential for competitive and meaningful state Senate elections in the state's most populous region.

Here’s how each of these districts would change under the plan:

Senate District 8 (now held by Darling):

Here’s the current district (in purple):

Here’s the proposed new district:

Under this plan, Darling loses the most Democratic parts of her district: Shorewood and fragments of Glendale and the city of Milwaukee. These areas are not just Democratic. They are intensely Democratic. They form the political base of the recall drive against Darling this year. The wards the Darling district sheds under the plan combined to give Republican Scott Walker a paltry 27% of the vote in last fall’s race for governor.

By contrast, the areas added to the Darling district under the new plan are in hugely Republican Washington, Waukesha and Ozaukee counties. These added wards combined to give Walker 73% of the vote last fall -- almost 50 points better than the areas they are replacing.

When you’re trading hostile territory for friendly turf, it’s pretty hard for an incumbent to do better than that. (All the numbers used here are based on my analysis of the new map; in some cases the figures aren’t exact because the new plan divides some existing wards.)

What’s the net partisan effect on this seat? Darling’s district supported Walker last fall 54% to 45%, which was marginally better than Walker did statewide. Under the new lines, Walker would have won the district by 27 points (63% to 36%) instead of nine.

In 2008, a very good Democratic year, Obama carried the Eighth and Darling nearly lost it, winning barely over 50% of the vote. These changes would lock the district down for the GOP even in bad Republican years.

Even if the new map were approved next week, the new lines would not be in effect for the August recall elections. But they would apply in 2012, meaning that if Democrats defeated Darling next month, their odds of holding the seat next year would be poor.

Senate District 5 (now held by Vukmir):

Here’s the current Vukmir district (labeled with a 5):

Here’s the proposed new district:

Vukmir’s district loses West Milwaukee and parts of West Allis and the city of Milwaukee under this map. She gains part of New Berlin and the part of Brookfield that she doesn’t already represent.

The wards she sheds voted Democratic in the 2010 governor’s race (53% for Tom Barrett, 46% for Walker).

The areas she gains voted overwhelmingly Republican, 70% to 30%.

The net effect on her seat? Walker won the current district by nine points, slightly better than his statewide performance. He would have won the new district by roughly 22. This is a district that elected a Democratic state senator in 2006, voted for Obama in 2008, and gave Vukmir a modest victory last fall in a huge Republican year. It would become a much safer GOP seat under the new plan.

District 21 (now represented by Wanggaard).

Here’s the current 21st district in light blue:

Here’s the proposed new district:

What the GOP plan does is take two competitive districts, one in Racine and one in Kenosha, and turn them into one safe Republican seat and one safe Democratic seat. Instead of two battleground seats that largely reflect county boundaries and combine Democratic-leaning cities with GOP-leaning suburbs, it would create one "blue" Racine-Kenosha urban district and one "red" Racine-Kenosha suburban district.

The Wangaard seat sheds most of the city of Racine and some nearby areas under the plan. These wards voted 63% Democratic in last fall’s race for governor. (The one section of the city of Racine that's kept in the Wanggaard district is the one where Wanggaard lives).

The district expands south and west into wards that combined to vote 63% Republican in the 2010 governor’s race.The new Wanggaard district also absorbs the Kenosha County neighborhood of Democratic state Sen. Bob Wirch, who is drawn out of his current district, the 22nd.

What’s the net effect on the partisan makeup of the Wanggaard seat? Walker would have won the new district by 23 points instead of nine, 61% to 38%.

This is a district that has elected state senators from both parties in recent years, voted blue for president and red for governor. Like the Darling and Vukmir seats, it would become a safe GOP seat under this plan, even in down cycles for the party, just as the neighboring 22nd District would become safe for Democrats.

Wisconsin has 33 state Senate seats. These changes alone would reduce the number of competitive Senate districts by four.