MILWAUKEE (Reuters) - Wisconsin voters will decide on Tuesday whether to throw Governor Scott Walker out of office in a rare recall election forced by opponents of the Republican's controversial effort to curb collective bargaining for most unionized government workers.

The rematch with Milwaukee's Democratic Mayor Tom Barrett, who Walker defeated in a Republican sweep of the state in 2010, is the end-game of six months of bitter fighting in the Midwestern Rust Belt state over the union restrictions Walker proposed and enacted.

The recall election in closely divided Wisconsin, which helped elect Democrat Barack Obama as president in 2008, is seen as a dress rehearsal for the 2012 U.S. presidential election in November.

The vote is also viewed as a test of strength between organized labor and conservative opponents, both of whom have poured money and effort into the contest.

"This is going to be an early indication of which way the political wind is blowing in Wisconsin," said Steven Schier, a political analyst at Carleton College in Northfield, Minnesota. "It has implications for the presidential race and national politics. Wisconsin could be a swing state."

On Monday night, Walker held a raucous election eve rally on Milwaukee's south side, where he said his administration had been a successful one.

"We've kept our word. We've kept our promises," Walker told some 200 supporters at a banquet hall.

About two dozen protesters briefly confronted Walker supporters in a parking lot outside the hall until police arrived to separate the two groups.

This will be just the third recall election of a governor in U.S. history, and follows weeks of vociferous protests by demonstrators who occupied the state capitol in Madison as Walker and fellow Republican lawmakers pushed through the union curbs in March 2011.

The law forced most state workers, including teachers, to pay more for health insurance and pensions, limited their pay raises, made payment of union dues voluntary and forced unions to be recertified every year.

The measure angered Democrats and unions, who gathered nearly 1 million signatures on petitions to force the recall election.

If Walker wins, Schier said observers will forecast Republican chances in the November 6 general election by measuring his margin of victory against his nearly 6-percentage-point win over Barrett in 2010.

Walker has led Barrett narrowly in most opinion polls leading up to the balloting, with very few voters undecided, so each side has mounted intense get-out-the-vote campaigns.

'UNFORGIVABLE'

Opponents of the union curbs charge Walker and fellow Republican lawmakers undercut workers' rights.

"I've never been member of a union, but what Walker did was unforgivable," said Michael Thom, 42, a carpenter in Columbus about 25 miles northeast of Madison, who said he is not affiliated with either party. "What the Republicans did amounted to an attack on the working class."

Passions are just as strong among Walker supporters who believe the changes were necessary to reduce a yawning budget deficit and curtail the power of government unions.

"Walker said he'd cut spending and he did, so I'm with him," said Paul Peardon, 40, who works for a construction equipment manufacturer near Milwaukee.

"Workers in the private sector have seen their wages and pensions affected by the recession," Peardon said. "It's only fair government workers should have to share the same pain."

Walker's lieutenant governor, Rebecca Kleefisch, and three Republican state senators also face recall elections on Tuesday. A fourth state senator targeted for recall resigned, and a candidate from each party is vying for her empty seat.

Walker has traveled the country and raised some $30 million for the campaign. More than half came from donors outside the state, including brothers David and Charles Koch, the conservative owners of the conglomerate Koch Industries. Barrett raised about $4 million.

In 2008, Obama won Wisconsin by 14 percentage points over Republican John McCain. Two years later, Republicans roared back, electing Walker to replace the outgoing Democratic governor, defeating veteran Democratic U.S. Senator Russ Feingold and taking over both houses of the state legislature.

The outcome of Tuesday's vote will be claimed as a momentum builder by either presumed Republican presidential nominee Mitt Romney or Obama, analysts predicted.

"Both presidential campaigns are watching this race closely," Schier said. "If Barrett wins I wouldn't be surprised if Obama shows up in Wisconsin within a week. If Walker wins, the same is true for Romney."

Obama Tweeted on Monday "It's election Day in Wisconsin tomorrow, and I'm standing by Tom Barrett. He'd make an outstanding governor." Romney has called Walker a "hero."

A loss for Walker would derail a rising Republican star who has knocked down talk he might become Romney's vice presidential running mate, whereas a Walker win could devastate unions that are a powerful Democratic constituency.

Only two previous recall efforts against sitting governors have ever made it to the ballot: Lynn Frazier in North Dakota in 1921 and Gray Davis in California in 2003. Both lost.

"If Walker wins the recall election he will be able to boast of being the only U.S. governor ever to be elected twice to the same office in one term," said Charles Franklin, a professor of law and policy at Marquette University Law School in Milwaukee.