The five main parties in the fray are Merkel's Christian Democrats (CDU), the Social Democrats (SDP) headed by her rival Frank-Walter Steinmeier, the Free Democrats (FDP), the Greens and the far-left Linke.
Having shared power with the SDP in a loveless grand-coalition since the last elections in 2005, Merkel is aiming to dump Steinmeier and forge a new alliance with the liberal Free Democrats.
But with one in four voters still said to be undecided on the eve of the vote and opinion polls suggesting a closer contest, Merkel may not get enough support to drop the Social Democrats as a coalition partner.
Steinmeier warned undecided voters at a rally in Dresden on Saturday that should Merkel succeed in forming a centre-right coalition government "she will take us backwards to the 1990s - with a few people at the top getting tax cuts, but the vast majority of us getting squeezed".
Rising unemployment, an increasingly troubled troop deployment in Afghanistan and several al-Qaeda threats make predicting a result difficult.
Barnaby Phillips, Al Jazeera's correspondent in Berlin, reported: "Either we will see a continuation of the grand-coalition ... or we will see a new centre-right continuation, with the SDP dropping away. In either case, we'll see more change in policy at home, rather than abroad.
"Foreign issues like Afghanistan have not really featured very much in the elections, certainly the main parties have not brought up an issue that has been unpopular.
"If we were to end up with the centre-right coalition, which is very possible, I think we will see a domestic change in taxation, and possibly adoption of more pro-nuclear policies ... but not dramatic changes with regards to relations with Europe, Russia or involvement in Afghanistan."
In contrast to many countries, Germans do not elect their leaders directly but vote for parties.
Each person casts two votes on a single ballot paper. In the first vote, people elect their chosen candidate in their electoral district to the Bundestag lower house of parliament and the winner takes up the district's seat.
In the second vote, people choose their preferred party from the 29 parties participating in the election by a complex system of proportional representation.
As no one party is usually strong enough to achieve a majority in the parliament on its own, coalition governments have ruled the country for more than 40 years.
The leader is then chosen by the coalition that holds the majority in the Bundestag.