HANOVER, Germany - German Chancellor Angela Merkel's Christian Democratic Union (CDU) party led in a key state election in Lower Saxony, exit polls showed on Sunday, though winner remained unclear.

Merkel's conservative CDU earned about 37 percent of votes, exit polls released by German ARD and ZDF television showed. Its junior partner in Lower Saxony, as well as at federal level, the pro-business Free Democratic Party (FDP) got about 10 percent.

The Social Democratic Party (SPD), CDU's main rival, claimed a support rate of about 33 percent, while its ally, the Green party, had about 14 percent.

The "red-green coalition" of SPD and the Greens was trying to oust the ruling "black-yellow coalition" of CDU and FDP both in Lower Saxony and in the federal government. But it would remain unclear which coalition would win until official result came out late Sunday night.

"It's an exciting election night, no one knows who will be the winner," said Stephan Weil, mayor of Lower Saxony capital of Hanover and SPD candidate for the state's new prime minister.

However, it was clear that the biggest winner in Sunday's election was the FDP. The junior party in ruling coalition had failed to reach the 5 percent threshold needed to re-enter the state parliament in previous polls. If that happened in Sunday's election, it was expected that the party's leader Philipp Roesler would have to resign.

The election in Lower Saxony was seen as a rehearsal for the federal election this autumn, and could affect campaigns for the next eight months.

If the ruling coalition in Lower Saxony succeed in holding power in the fourth biggest German state by population, it would be a strong boost for Merkel's campaign for her third term.