Angela Merkel is in a fight to clinch a new term for her ruling coalition despite enjoying overwhelming popularity and leading an economy that is the envy of Europe.
Polls ahead of Sunday's national election show her centre-right alliance on a knife-edge as her junior partner's support slumps.
Ms Merkel and her conservative Christian Democratic Union appear likely to fend off a challenge from centre-left rival Peer Steinbrueck and emerge as the biggest party in the lower house of parliament, whose members choose the chancellor - making her the strong favorite to win a third term.
But no single party has won an absolute majority in Germany in more than 50 years. And surveys show Ms Merkel's coalition partner, the pro-business Free Democratic Party, has fallen from the nearly 15% support it won in 2009 to about the 5% level needed to keep any seats in parliament.
If Ms Merkel's alliance falls short of a parliamentary majority, the likeliest outcome is a switch to a Merkel-led "grand coalition" of her conservatives with Mr Steinbrueck's Social Democratic Party, the same combination of traditional rivals that ran Germany from 2005 to 2009 in Ms Merkel's first term.
That is unlikely to produce a radical change in government policies. However, it could signal a subtle shift in emphasising economic growth over the austerity that Germany has insisted on in exchange for bailing out economically weak European countries such as Greece.
Final results are due within hours of polls closing. But with margins so close, the country could still face weeks of protracted horse-trading before a clear picture emerges about the make-up and policies of Germany's next government.
Ms Merkel's centre-right coalition might win re-election but "it will be very tight," said Oskar Niedermayer, a political science professor at Berlin's Free University.
Much may depend on the turnout among the nearly 62 million voters - about 70% in the 2009 ballot. In the days before the voting, prominent figures from all major parties have urged supporters to vote, with the projected outcome too close to take any chances.