State elections test Merkel's hold on power in Germany
Germans are voting in a state election in Baden-Wuerttemberg, which analysts say will test Chancellor Angela Merkel's hold on power.
Mrs Merkel's Christian Democrats could be ousted in the state for the first time since 1953, opinion polls suggest.
They say that the Social Democrats and the Greens have enough support to form a governing coalition.
The opposition to Mrs Merkel's party has also been energised by anti-nuclear sentiment.
On Saturday, tens of thousands of Germans took part in what are thought to be the country's biggest-ever protests against nuclear power, in the wake of the Fukushima nuclear crisis in Japan.
Japan is still struggling to stabilise the Fukushima plant, crippled by the earthquake and tsunami of 11 March.
Polling stations across Baden-Wuerttemberg opened at 0600 GMT and will close at 1600 GMT. About eight million people are eligible to cast their votes in one of Germany's industrial powerhouses.
Recent polls have suggested that the Social Democrats and Greens could secure 48% of the vote.
Mrs Merkel's party and the Free Democrats, a coalition partner, are forecast to win about 43%.
The BBC's Germany correspondent Stephen Evans says that two specific issues have driven discontent against the Christian Democrats: nuclear power, particularly after the disaster in Japan, and a plan for a big railway project which could transform the centre of Stuttgart.
Mrs Merkel changed policy on nuclear power a week ago, suspending for three months an earlier decision to extend the lifetime of Germany's nuclear reactors. Four of them are based in Baden-Wuerttemberg.
The chancellor also temporarily shut off the seven oldest reactors pending a safety review.
But one of Mrs Merkel's ministers said in a leaked memo that such decisions made on the eve of elections "were not always rational".
Germany - which was hit by fallout from the 1986 Chernobyl nuclear disaster - has long had a large body of public opposition to nuclear power.
Our correspondent also says that some voters in the state say they feel their views have been ignored in favour of those of bigger corporate interests.
Preliminary results are expected soon after the voting ends.