Polls opened on Sunday morning, with Angela Merkel looking assured of a fourth-term as Chancellor.
But a large number of people look set to vote for the anti-Muslim AfD, a swing which could see far-right politicians enter the Bundestag for the first time in over half a century.
The party is currently polling at nearly 13 per cent and could end up with nearly 100 representatives, making them the main opposition to any coalition Mrs Merkel puts together.
Writing for the newspaper Bild, President Frank-Walter Steinmeier of the Social Democrats, appeared to warn AfD supporters could triumph if other voters stayed at home.
He wrote: “It has perhaps never been as clear that the elections are about the future of democracy and Europe.
“If you don’t vote, others decide.”
Both Mrs Merkel and her main challenger, Social Democrat leader Martin Schulz, are concerned that a low turnout could work in favour of smaller parties like the AfD.
In Germany’s proportional election system, low turn-out can boost smaller parties, giving them more seats from the same number of votes.
Last year’s regional elections saw Merkel’s party suffer setbacks to the AfD, which profited from resentment at her 2015 decision to open German borders to more than one million migrants.
Mr Schulz, who has described the AfD as “gravediggers of democracy” “a party of haters” and, yesterday, also urged people to vote yesterday.
He said: “Young people, think about Brexit. Think about Trump.
“Go vote. Take this right to vote seriously, and use it.”
In a bid to challenge the right-wing surge, Mrs Merkel’s interior minister, Thomas de Maiziere, promised government would combat Islamic terrorism by strengthening European borders and bolstering security at home.
He criticised the AfD as “a wolf in sheep’s clothing” and said Germany’s domestic intelligence agency was studying “whether right-wing extremists are seizing power and exerting influence on the party”.
The AfD was founded in 2013 with the original goal of opposing large bailouts of financially strapped euro zone countries but from 2015 shifted its focus to immigration.
Mainstream parties have ruled out governing with the AfD.
It has come under increased scrutiny in recent weeks after its top candidate Alexander Gauland said the German integration minister should be dumped in her parents’ homeland of Turkey, and that Germans should be proud of what their military did in World War One and Two.
Jewish and Muslim groups say the AfD’s rhetoric has opened the door to more hate speech and anti-Semitism.
An INSA poll published by the Bild newspaper on Saturday suggested that support was slipping for Mrs Merkel’s conservatives, who dropped two percentage points to 34 per cent, and the SPD, down one point to 21 per cent.
The two parties currently govern Germany in a “grand coalition”, an unwieldy arrangement which may not be able to reform this time around.
Mrs Merkel may instead opt to form a “Jamaica coalition” with the pro-business Free Democrats (FDP) and Greens.
Voting opened at 8am local time on Sunday, with polls closing at 6pm.