Madison, Wisconsin (CNN) -- Polls closed Tuesday night in Wisconsin's recall elections as six Republican state senators who supported Gov. Scott Walker's efforts to hobble public employee unions fought to hold onto their seats.
Tuesday's races stem from winter's bitter battle that saw pro-union protesters camping out in the state capitol and Democratic senators fleeing the state in an unsuccessful attempt to halt Walker's legislation. More than $30 million has been spent during the recall campaigns, most of it from out of state.
Early, unofficial returns published online by the Milwaukee Journal-Sentinel showed GOP incumbents Robert Cowles, Alberta Darling and Sheila Harsdorf with comfortable leads over their Democratic challengers. Sens. Luther Olsen and Randy Hopper appeared to be leading closer contests, while one Democrat, Jennifer Shilling, had pulled slightly ahead of state Sen. Dan Kapanke.
Democrats are angling to win GOP-held seats to capture control of the upper chamber, which Republicans currently hold 19-14, and build momentum for a planned effort to recall Walker.
"If the Democrats pick up three or four or, less likely, five seats, that will surely energize the forces that want to recall Gov. Walker next year," University of Wisconsin political science professor Charles Franklin told CNN. "If they fall well short of that, then it's an open question as to whether they see that as worth pursuing or not."
Walker, who was elected in November with strong support from tea party activists, can't be recalled until after he serves a year in office.
Republicans have countered with their own recall attempts against three Democrats. One of them survived a challenge in July, while two others will be on the ballot next week.
Ken Goldstein, president of Campaign Media Analysis Group and CNN's consultant on TV advertising, said the amount of money flowing into the races is "extraordinary."
"To see this much spent in Senate elections and to see this much spent on broadcast TV is absolutely unprecedented," Goldstein said. "The races have become nationalized and the perceived stakes are high, so there is lots of money to play with."
Walker set off a firestorm in January when he moved to curtail the collective bargaining rights of most state employees. With majorities in both houses of the Legislature, Walker and his GOP allies voted to limit raises for public employees except police and firefighters to the rate of inflation, bar unions from deducting dues from workers' paychecks and force them to hold a new certification vote every year.
Republicans insist that the legislation was necessary to control skyrocketing public employee benefit costs and close a budget shortfall, while Democrats called it an attempt to gut public-sector labor unions, one of their core constituencies. The state Supreme Court upheld the legislation in June.
Unions have vowed to make an example of the GOP senators, while tea party groups joined forces Friday for a four-day bus tour defending them. While Democrats won't be able to roll back the union restrictions with control of only one chamber of the Legislature, they would be able to block any of Walker's other initiatives.
CNN's Ted Rowlands, Gabriella Schwarz, Paul Steinhauser and Ashley Killough contributed to this report.