In the end game of a tight election race, German Chancellor Angela Merkel promised voters Friday they will be in safe hands if she stays leader of Europe's economic giant.

"Germany has had four good years," she wrote in a letter mailed to five million households ahead of Sunday's election, in which she seeks a third term for her Christian Democratic Union (CDU).

"We have achieved a lot together. I also want the next four years to be good," wrote Merkel, who is dubbed "Mummy" in her country and often called the world's most powerful woman.

"If you want me to keep working as your chancellor, then please give your votes to the CDU on Sunday," said the letter signed "Your Angela Merkel".

Latest polls gave popular Merkel's coalition with the pro-business Free Democrats (FDP) a strong lead over an alliance of the centre-left Social Democrats (SPD) and ecologist Greens.

The margin narrowed to a razor-thin 45.5-44.5 percent against the wider left opposition -- also including the far-left Linke, which has been rejected as a coalition partner by all other parties.

"We should expect a government in which the conservatives are the strongest political force and Mrs Merkel very likely the chancellor," said Gero Neugebauer, a political scientist with Berlin's Free University.

The big question is with whom Merkel may end up governing.

A wildcard factor are small parties which have hovered close to the five-percent mark required for entry into parliament -- including the FDP and the new eurosceptic AfD, which scored four percent in the latest poll for ZDF public television.

Merkel's biggest liability are her junior coalition partners, who are fighting for political survival after being kicked out of Bavaria's state parliament in elections last week.

The FDP, in a desperate bid to stay in power, has urged conservatives to "split" their ballot by casting district votes for the CDU but giving the Free Democrats their second or party vote to save the coalition.

Merkel in her letter implored her supporters to do no such thing and avoid what one newspaper dubbed a "blood transfusion" for the struggling FDP.

Should the smaller party crash out, Merkel would have to seek new partners -- most likely her party's traditional rivals the SPD, with whom she ruled in a previous "grand coalition" from 2005 to 2009.

For now the two big parties remain in battle mode, seeking to energise their base and win over millions of undecided voters.

Merkel was headed to Munich later Friday on the eve of the world-famous Oktoberfest beer festival, days after the CDU's regional sister party scored a huge victory in a state election.

Her challenger, the SPD's Peer Steinbrueck, at a Berlin rally Thursday made a spirited call for more social justice and solidarity in Europe as well as a more muscular leadership style than what he labelled Merkel's "going around in circles".

The self-styled "straight talk" candidate has suffered through a series of gaffes and blunders, not helped by a magazine cover photo that showed him giving the middle finger as a non-verbal reply to a question on his limping candidacy.

The image heightened the contrast between brash Steinbrueck and what has been called the sphynx-like persona of Merkel, who often sits out problems and avoids polarising language.

The chancellor's campaign trail symbol -- used in a giant placard at Berlin railway station -- is the diamond gesture she makes with her index fingers and thumbs in many photos.

One supermarket chain Friday took out a full-page newspaper advertisement that showed a mock ballot paper with pictures of two hand gestures -- one making the diamond, the other flipping the bird. "If in doubt," it said, "choose the diamond."

Neugebauer criticised what is widely seen as a shallow campaign focused on symbols not substance.

"There is no discussion about the great problems, such as the social split in German society or the problems that exist in Europe," he said.

"When you look at the election campaign, and the likelihood of a grand coalition, you have to conclude that the political culture in Germany has pretty much gone to the dogs."