Republicans are taking aggressive steps to protect House freshman Sean Duffy under a draft redistricting plan that hasn’t been released to the public but is circulating among Wisconsin members of Congress.

The plan goes to significant lengths to make Duffy’s marginally Democratic seat more Republican, according to a map provided by a Democratic source and whose main features were confirmed by sources in both parties. (You can find the map here).

Arguably his party’s most vulnerable House incumbent, Duffy represents the Seventh congressional district in northern Wisconsin, previously held by Democrat Dave Obey.

The plan would carve out a Democratic chunk of central Wisconsin (including the cities of Stevens Point and Wisconsin Rapids) and shift it from Duffy’s seat to Democrat Ron Kind’s western Wisconsin district.

The result: Duffy would “lose” one of the most Democratic counties in the state, Portage, while gaining the fastest-growing Republican county in the state, St. Croix.

Members of Wisconsin’s congressional delegation discussed the map behind closed doors Monday evening. GOP House member Paul Ryan took the lead among his colleagues in crafting it, according to sources.

The plan is not yet official and could change before it's adopted. It will be up to the Republican-controlled state Legislature to settle on the final lines, part of a high-stakes, once-in-a-decade redrawing of congressional and legislative districts that parties and incumbent lawmakers use to improve their election prospects.

Unlike ten years ago, Wisconsin is not losing a US House seat, so dramatic changes aren’t required to draw the new map. But the state has to adjust its congressional lines to reflect population shifts that have occurred since 2000.

The three congressional districts that border Illinois (the First, Second and Third) have gained the most people since 2000 and need to shed territory in order to equalize populations among the eight districts.

The other five need to gain territory.

The map now under consideration makes some notable changes across the state.

In the illustration below, the current district lines are in white, and the proposed new lines are outlined in red:

Milwaukee County would be divvied up among four different congressional districts, compared to three now and two before 2002.(In other words, half the state's US House seats would include a part of Milwaukee County).

A small slice of the county's north shore, along with Ozaukee County, would be absorbed by the Sixth District now held by Republican Tom Petri of Fond du Lac.

Most of the North Shore suburbs, such as Shorewood, Glendale and Whitefish Bay, would shift from Republican Jim Sensenbrenner’s Fifth District to Democrat Gwen Moore’s Fourth District.

And Greenfield in southern Milwaukee County would move from Ryan’s First District to Sensenbrenner’s. Sensenbrenner’s district would also absorb all of West Allis, now divided between his and Moore’s district. As noted above, Sensenbrenner would no longer represent Ozaukee County, which would be in a separate US House district from neighboring Washington County. And he would cede more of Waukesha County to Ryan.

The Fox Valley would also see changes. Republican Reid Ribble’s Eighth District would dip southward into Calumet County and part of Winnebago.

Democrat Tammy Baldwin’s Second District would shift westward, though still anchored by the Democratic bastion of Dane County.

But the biggest changes involve the Duffy and Kind seats, which would both become better for the incumbents.

To make Duffy’s seat more Republican, the plan takes a section of central Wisconsin that has been part of the Seventh district for decades, and connects it to Kind’s Mississippi River district with a corridor through Adams County.

Portage was the eighth-most Democratic County in Wisconsin in the most recent elections for President and governor. President Obama carried it by 28 points.

The 7th district would also shed the city of Chippewa Falls in Chippewa County.

Meanwhile, Duffy’s district would gain GOP turf in northeastern Wisconsin.

And in western Wisconsin it would extend south into St. Croix, a fast-growing county that has trended sharply Republican in recent years; Republican Scott Walker carried it by 25 points in 2010.

It’s impossible to quantify how much more Republican the Duffy district would become or how much more Democratic the Kind district would become without knowing the exact municipal and ward boundaries in the plan, which weren’t available Monday.

The partisan effects of the changes do not appear to be massive. But for Duffy, a GOP freshman trying to defend a traditionally Democratic seat, the proposed new lines could make a difference in a close election.

The non-partisan Cook Political Report currently rates Duffy’s district as “D+3,” meaning Democrats have a built-in three-point partisan advantage based on its election history.

Obama carried it by 13 points in 2008, slightly under his statewide margin. Democrat John Kerry carried it by 1.4 points in 2004, slightly better than his statewide margin.

Republicans in the legislature haven’t said when they plan to act on redistricting, but some Democrats expect them to move fairly quickly to pass a plan before the August recall elections, which could cost the GOP control of the state Senate if Democrats gain at least three seats.