BERLIN - Voters in three German states headed to the polls on Sunday for regional elections that are seen as a key test of sentiment before a federal vote next month in which Chancellor Angela Merkel hopes to win a second term.

Merkel and her conservative Christian Democrats (CDU) hold a comfortable 12-15 point poll lead over their center-left rivals, the Social Democrats (SPD), ahead of the September 27 national vote and she appears to be cruising toward re-election.

But the SPD sees the state votes -- in Saarland, Saxony and Thuringia -- as a chance to slow her drive for a second term. A poor performance by the conservatives on Sunday would give the SPD a psychological boost for the final phase of the campaign and sour the mood in Merkel's camp.

Any erosion in support might endanger her hopes of sealing a center-right government next month with the business-friendly Free Democrats (FDP) and possibly force her into another awkward grand coalition with the SPD.

The SPD will try to make the most of any gains in these regional votes, said Peter Loesche, emeritus professor of political science at Goettingen University.

Voting began at 8 a.m. (0600 GMT) and the first exit polls are due to be published at 6 p.m. (1600 GMT). Due to unusually complex coalition mathematics and policy differences, it could take weeks until new governments are in place.

Conservative allies of Merkel currently govern in all three of the states that will vote on Sunday and opinion polls give each of them a lead. But in Saarland, on the French border, and Thuringia, in eastern Germany, victory is far from assured.

In both states, three-way coalitions of the SPD, far-left Linke and environmentalist Greens could sweep the incumbents from power if they can overcome their differences.

If this happens in Saarland, a tiny state of a million people tucked into a western corner of Germany, it would be the first regional partnership of the SPD and Linke, or Left party, in the west of the country.

Direct descendants of former East Germany's ruling communist party, which built the Berlin Wall, the Left party is loathed by many moderates in the SPD and cooperation with them outside the country's eastern regions has been taboo until now.

That has forced the SPD leadership into a delicate balancing act -- condoning cooperation with the Left at the regional level while ruling it out nationally after next months federal vote.

Merkel's conservatives are likely to seize on any signs the SPD is ready to work with the Left to stir up voter fears of a dangerous red wave.

In the third state of Saxony, Merkel's conservatives look poised to hold onto power by either continuing their coalition with the SPD or sealing a new partnership with the FDP.

After scoring 9.2 percent in the last election in Saxony, the far-right NPD could fail t reach the five percent threshold required to make it into the state assembly.