Northern Virginia’s rapidly growing outer suburbs would gain one new seat in the state Senate and three in the House of Delegates under proposed maps for the once-a-decade redistricting released by legislative leaders Tuesday night.

The new districts are driven by the region’s rocketing population growth over the last decade and represent a shift in power from southern portions of the state that have grown more slowly.

The General Assembly must realign the boundaries of its 40 Senate seats and 100 delegate districts once every 10 years to reflect new census numbers.

The proposals released Tuesday were drawn up by Democrats in the state Senate, where their party holds a thin two-vote majority. Likewise, the House map was drawn by Republicans who hold a commanding majority in the chamber.

The two chambers will also have to agree to new boundaries for the state’s 11 congressional districts; state leaders will release plans for those districts in coming weeks.

Each side used sophisticated mapping software to draw districts designed to protect their incumbents and consolidate their party’s majorities.

The Senate proposal, sponsored by Sen. Janet D. Howell (D-Fairfax), deals with lower population counts in the Hampton Roads area by collapsing the districts of two incumbent Republicans in Virginia Beach. It also merges two underpopulated Republican districts in Southside Virginia.

“Senate Democrats have crafted an outrageously partisan redistricting plan that will go down as one of the most notorious examples of gerrymandering in history,” said Senate Minority Leader Thomas K. Norment (R-James City).

In Northern Virginia, senators proposed divvying up the city of Alexandria, portions of which had been included in two districts, into three districts, allowing its heavily Democratic voting base to temper more a politically integrated district in Prince William.

The new district in Northern Virginia would include portions of western and southern Loudoun County, as well portions of Clarke, Fauquier and Prince William counties. The Richmond suburbs would also gain a new Senate seat.

Top senators said that their maps comply with state and federal law and that they worked to keep communities together — noting that their plan splits 43 counties and towns, only two more than the current maps.

Majority Leader Richard L. Saslaw (D-Fairfax) said Democrats attempted to shore up some of their districts but left a competitive map, noting that U.S. Sen. James Webb (D) won 21 of the map’s 40 districts in 2006, reflecting Webb’s narrow win statewide that year.

“The theme here is that we didn’t get greedy,” he said.

There will be particular interest in the Senate lines because Republicans plan an aggressive campaign to win back the chamber from Democrats, who have held it since 2007.

In the House, Republican leaders collapsed the underpopulated district of the leader of the Democratic opposition, merging the Southside seat of House Minority Leader Ward L. Armstrong (Henry) into the district of a neighboring Republican.

“They’re doing it exclusively to get at me,” Armstrong said. “I’ve made a lot of people mad in Richmond.”

They added three House seats to Northern Virginia: Two seats largely centered in the growing counties of Loudoun and Prince William and one that includes another portion of Prince William as well as a portion of Stafford.

They accomplished that shift by combining two districts in southwest Virginia, collapsing four districts in the Hampton Roads area into two and combining two seats west of Loudoun.

“It’s a fair map,” said Del. S. Chris Jones (R-Suffolk), who is sponsoring the Republican plan. “We received input from both sides of the aisle, including the black caucus.”

The release of the maps on Tuesday night is the official start of the redistricting process. Legislative committees in charge of the process will hold public hearings around the state and the General Assembly will convene in Richmond on Monday for a special session on redistricting.

This is the first time since Reconstruction that redistricting will be tackled by a divided legislature. But the General Assembly must solve any disputes rapidly or risk disrupting this year’s November elections. Virginia is one of only four states that will hold a state vote in the fall.

Also speeding the process: Virginia is one of nine states that is required under the terms of the 1965 Voting Rights Act to submit its plans for approval to the Justice Department, to ensure the proposals do not dilute the power of black voters.

The speed required in the process will provide a powerful incentive for legislative leaders to give the opposite party a wide berth to draw their own lines. But they will face challenges from the minority party in each chamber, as well as citizen groups working to eliminate partisan gerrymandering.

In the Senate, Republicans submitted a competing map, which Norment said was more fair.

Courts have interpreted the Voting Rights Act to require that Southern states maintain districts where a majority of residents are black, to ensure those residents’ voting power is not diffused.

In keeping with the law, the Senate map includes five majority-minority districts — the same as a decade ago. The House plan likewise includes 12 majority-minority seats.

After the General Assembly approves its new districts, they will go to Gov. Robert F. Mc­Don­nell (R) for his approval.

Mc­Don­nell has established a bipartisan advisory commission on redistricting, which will meet Wednesday to recommend alternative maps drawn without regard to party or incumbency. Groups that sought the commission’s creation will urge Mc­Don­nell to use his veto pen to ensure the final plan reflects the commission’s suggestions.

“There’s no reason to believe anything we’ve done is going to impact the legislature,” said Richmond businessman E. Bryson Powell, a member of a group advocating redistricting reform. “The governor has to buy into the final plan.”