MADISON, WIS. - Tuesday brings a series of recall elections unprecedented in the history of the state or nation.

Since 1908, there have been 20 recorded state legislative recall elections held in the United States, according to one recall expert.

Wisconsin is in the process of holding nine such elections in the space of a month.

With control of the Wisconsin Senate in the balance, six Republican state senators will face a recall vote Tuesday.

One Democratic state senator has already weathered a recall attempt, and on Aug. 16 two more Democrats in the Senate will be up for recall.

"Wisconsin has taken a quantum leap in a fast-changing process, in what has been a growing use of the recall" nationally, said Joshua Spivak, who writes the Recall Elections Blog.

There is intense interest -- from voters on the street to national political groups -- in the elections sparked by lawmakers' positions on GOP Gov. Scott Walker's law ending most union bargaining for most public workers.

Tens of millions of dollars are being spent on the recall campaigns by candidates and independent groups.

President Obama's Organizing for America was pulling together canvassing drives Monday, and the Tea Party Express was concluding a bus tour of the state with rallies in Rhinelander and De Pere.

State Government Accountability Board officials are not releasing turnout projections because of the unusual, difficult-to-predict nature of the election.

But people on both sides of the races were predicting healthy turnouts.

Fond du Lac City Clerk Sue Strands estimated an exceedingly high turnout Tuesday of 75 to 80 percent of registered voters in her area, where Republican state Sen. Randy Hopper faces a challenge from former Oshkosh Deputy Mayor Jessica King, a Democrat. Strands said the volume of calls, voter registrations and absentee ballot requests that her office has been receiving is comparable to those in a gubernatorial race.

Joe Heim, a political science professor at the University of Wisconsin, La Crosse, said both Democratic and Republican voters have gotten more engaged as a result of the state's political climate.

"It's extremely historical and unprecedented," Heim said. "[Republicans] woke up a sleeping giant and energized a group of people that had not been ... particularly active in politics in recent years."

John Hogan, who is managing Senate Republicans' recall efforts, acknowledged the national implications of the elections and said voters would support GOP senators for helping to cope with the state's budget problems.

"The nation is watching. What you have here is a state making a decision on which way do you go," Hogan said. "We need legislators to make the tough decisions."