BERLIN - A state election on Sunday in western Germany offers Chancellor Angela Merkel's conservatives a tough test against their resurgent center-left rivals six months before Merkel seeks a fourth term in a national vote.

The election for the state legislature in Saarland, a region of just under 1 million people on the French border that Merkel's Christian Democrats have led since 1999, is the first of three regional votes before Germany's Sept 24 national vote.

It's being watched closely as the first electoral test since the center-left Social Democrats nominated Martin Schulz as Merkel's challenger in January.

Schulz, a former president of the European Parliament but a newcomer to national politics, has boosted his party's long-moribund poll ratings and injected it with new self-confidence.

He has lifted party support by 10 percent with promises to help the socially disadvantaged and end Merkel's almost 12-year reign.

That boost means that a fourth Merkel term no longer looks inevitable and it also has tightened the race in Saarland.

Conservative governor Annegret Kramp-Karrenbauer until recently looked certain to win a new five-year term. But Social Democrat rival Anke Rehlinger now hopes to finish first, and polls suggest she could win a majority for an alliance with the opposition Left Party.

The two women currently govern together in a "grand coalition" of the biggest parties, an alliance similar to Merkel's at the national level.

Kramp-Karrenbauer is one of only five conservative governors in Germany's 16 states. Losing would be a worrying signal for the national campaign and for two bigger elections in May in states both led by the Social Democrats.