German Chancellor Angela Merkel, poised to win a fourth term in tomorrow's election, and her centre-left challenger Martin Schulz urged undecided Germans to use their votes with a third of the electorate still undecided.

Ms Merkel is widely expected to cruise to re-election with Mr Schulz's Social Democrats trailing by double digits but the far-right Alternative for Germany (AfD) could emerge as the third largest party, complicating the outlook for her next coalition.

A new INSA poll published by Bild newspaper showed sliding support for Ms Merkel's conservatives, who dropped two percentage points to 34%, and the SPD, down one point to 21% - both now joined in an unwieldy "grand coalition".

The anti-immigrant AfD, meanwhile, rose two percentage points to 13%, cementing its bid to be the first far-right party to enter parliament since the end of World War Two.

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Ms Merkel urged supporters to drum up votes by focusing on conservatives' efforts on behalf of families, a pledge to avoid tax increases and emphasis on increasing security in Germany.

The Christian Democratic leader also lauded the role of the European Union in providing stability in "a troubled world".

"We want to boost your motivation so that we can still reach many, many people today. Many are still undecided," Ms Merkel said.

In the western city of Aachen, Mr Schulz pledged to fight for every vote until polls close at 6pm tomorrow. He said high voter turnout was vital to offset growing support for the AfD, whom he described as "a party of haters".

"Young people, think about Brexit. Think about Trump," he said. "Go vote. Take this right to vote seriously, and use it."

Mr Schulz said the SPD overcame resistance from conservatives in their coalition to push through a minimum wage, same-sex marriage and other social priorities.

Mr Schulz asserted that Ms Merkel was "a world champion in not deciding," someone who simply parroted others' ideas.

He vowed to push for further reforms, including better elderly care facilities, affordable housing, an end to discriminatory practices that harm children of migrants, and free childcare.

First elected in 2005, Ms Merkel remains popular in Germany but has regularly faced jeers from left- and right-wing demonstrators at rallies during this campaign.

In Munich yesterday, Ms Merkel defended her 2015 decision to admit almost a million asylum seekers on humanitarian grounds, but pledged to prevent a repeat of that crisis.