The Virginia Senate is expected to clear a controversial congressional redistricting plan Friday and send it to Gov. Robert F. McDonnell (R) for his signature, but the long-debated lines still face hurdles before they can be put into place.

Drawn to reflect the results of the 2010 Census, the Republican map was approved by the state House last week for the second time. The same plan passed the chamber last year but was never approved by the Democratic-controlled Senate, which chose to advance its own set of lines.

Now, with Lt. Gov. Bill Bolling (R) positioned to cast tie-breaking votes in the 20-20 Senate, Republicans are poised to advance their map, probably on a party-line tally. But Democrats have sued, saying that the General Assembly violated the state constitution by failing to complete the task in 2011.

That case is pending in state and federal courts, even as the March 29 filing deadline looms for congressional candidates to make the ballot. The primary is set for June 12.

The two parties have traded blame over the impasse for months, with Democrats accusing Republicans of stalling last year in expectation of winning the Senate, and Republicans suggesting that Democrats want to have unelected judges draw them a more favorable map.

“At the end of the day, I believe it would be very disappointing and a great tragedy . . . that we would totally abdicate to” the courts, said Sen. Jill Holtzman Vogel (R-Winchester), the sponsor of the bill.

However, Sen. A. Donald McEachin (D-Richmond) said Republicans were “the ones who said they would not compromise, so for them to suggest that we were the holdup is a complete untruth.”

The Republican map would fortify most incumbent lawmakers, probably preserving the state’s congressional delegation split of eight Republicans and three Democrats. In Northern Virginia, the map would be especially helpful to Reps. Frank R. Wolf (R) and Gerald E. Connolly (D), each of whom would shed some difficult political territory and gain friendlier turf.

Senate Democrats contend that the plan dilutes minority voting strength by “packing” more African Americans into the 3rd District of Rep. Bobby Scott (D) while spreading the remainder of the state’s minority voters too thinly too make a difference elsewhere.

“The population of Virginia is such that we ought to have more than one minority member of Congress,” said Del. Jennifer McClellan (D-Richmond) during last week’s House debate.

The plan advanced last year by Senate Democrats took a different approach, most notably by reducing the number of African American voters in Scott’s district while creating a second seat, held by Rep. Randy Forbes (R), with a significant minority population.

In November, a half-dozen Virginia voters filed suit in state and federal courts asking judges to draw congressional lines for 2012 because legislators failed to do so in a timely way. The Virginia Constitution, they noted, says that the General Assembly “shall draw electoral districts . . . in the year 2011 and every ten years thereafter.”

The suit got a hearing in state Circuit Court in Richmond last week, and a hearing in the federal case is scheduled for Feb. 10 in the U.S. District Court for the Eastern District of Virginia in Alexandria. In both venues, the office of state Attorney General Ken Cuccinelli II (R) is seeking to have the case dismissed.

Republicans are making two arguments: Past Virginia court cases have established that the word “shall” in the constitution doesn’t mean “mandatory,” and even if the General Assembly did violate the constitution, a map drawn by the legislature is still preferable to one drawn by judges.

“A court is highly unlikely to usurp the legislature’s authority to legislate a reapportionment plan on the basis that they missed their window of opportunity by one month,” said Lee Goodman, who serves as general counsel for the Republican Party of Virginia.

Once the Senate passes the Republican map and McDonnell signs it into law, Goodman predicted, “I think the lawsuit will be deemed moot .”

But Jeffrey M. Wice, a redistricting attorney who has represented Virginia House Democrats, said that while “courts often prefer to leave drawing maps to legislatures, courts also have to interpret the constitution in a way that follows the law. And the Virginia legislature has clearly run afoul of the state constitution.”

Under the Voting Rights Act, Virginia’s map must also receive “pre-clearance” from the Justice Department before it can be put into place to ensure that it does not violate the rights of minorities. That process can take as long as 60 days, though lawyers on both sides of the issue said Justice can move more quickly if necessary.