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Mr. Baker, who has taken heat in recent days for calling the federal health care overhaul the “domestic equivalent of the Iraq War,” has been a sharp critic of Senate Democrats’ redistricting plan, which Mr. Barker in large part helped craft.

“There’s gerrymandering,” Mr. Baker said, “then there’s gerrymandering on steroids.”

Still, Mr. Barker says his party split fewer precincts than the Virginia General Assembly did 10 years ago when Republicans controlled the contentious process.

“Every outcome is consistent with where the district is,” he said.

Any of the four races — as well as several others — could help determine whether Democrats retain their 22-18 Senate majority.

Republicans are solidly in control in the House of Delegates, holding 59 seats while two independents caucus with them. But the outcome of several races Tuesday could have significant consequences.

Democrat Pamela Danner is taking on Republican Barbara Comstock, who some see as a potential successor to Rep. Frank R. Wolf, in the 34th District, which Ms. Comstock won by 422 votes over Democrat Margaret G. Vanderhye in 2009.

In Southside, House Minority Leader Ward L. Armstrong, who was redistricted out of his Henry County seat, moved to his nearby mother-in-law’s home to take on Delegate Charles D. Poindexter in the new 9th District.

In what has become one of the most heated — and expensive — delegate races, Mr. Armstrong, who has expressed interest in running for statewide office, has raised more than $1 million with Mr. Poindexter nearly matching that amount.