While polls say Merkel's Christian Democrat (CDU) party will get the most votes in Sunday's balloting, parliamentary partner the Free Democrats might not get enough votes to keep their seats, the BBC reported.
The Free Democrats (FDP) received 15 percent of the vote in the last election in 2009, but polling indicates this time the party may not get the 5 percent necessary to retain representation in the federal legislature.
If the polls are correct, Merkel will have to look for new coalition partners.
One of them could be the Green Party, which in a rally in Berlin criticized the chancellor's reversal on her pledge not to raise taxes.
"Then they ended it with the car trade-in rebate scheme that was environmental, industrial and transport policy madness," said the party's parliamentary co-chair Juergen Tritten.
Another possible partner is the Alternative fuer Deutschland, a new party formed largely from disaffected CDU members. However, the AfD is staunchly anti-euro, possibly creating an unhappy political marriage.
Germany is a prominent member of the euro zone. It has the largest population in the European Union and one of the highest GDPs on the continent.
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