Merkel fell short of scoring an absolute majority in the 630-member Bundestag and must now build either a "grand coalition" with the SDP, an odd-couple relationship that was the hallmark of her first time in office in 2005, or negotiate a first-ever governing alliance with the Greens.
The leader of the SDP offered Merkel no immediate helping hand. "The SDP will do itself good if it doesn't speculate about what a future government could look like," the party's candidate for chancellor, Peer Steinbrueck, told supporters. "The ball is in Merkel's court. She now has to find a majority."
Steinbrueck served as Finance Minister in the grand coalition that took office in 2005 but says he will not go into office with Merkel again. Others in the party are more positive towards the idea. Juergen Trittin, the Greens' main candidate, has not completely ruled out going into coalition with Merkel although some in the party fiercely oppose serving alongside the strongly conservative CSU.
The huge personal victory puts the 59-year-old Chancellor in a strong position in any scenario. She now joins her conservative predecessors, Konrad Adenauer and Helmut Kohl, as one of Germany's three-term chancellors. If she completes her new four-year spell, she would surpass Margaret Thatcher, who served 11 and a half years as British Prime Minister, as the longest-serving woman leader in Europe.
Endorsement by the voters of Europe's paymaster also boosts her status in the eurozone crisis, where she has led a quiet yet determined charge for governments to reduce debt and honour their pledges to the single currency.
"Whatever happens, this is a massive public endorsement for Angela Merkel, who has established herself as the most powerful female politician ever. Part of her resounding victory must also be seen as a validation of her eurozone policy - expect more of the same," said Mats Persson, director of the thinktank Open Europe.
The financial markets will be closely watching in the coming days to see if forming the next government goes smoothly and if Merkel faces domestic constraints on such issues as the minimum wage and labour laws.
In 2005, it took the "grand coalition" 26 days to agree on the various posts and two months for the coalition to be officially formed and ratified.
The markets will also scrutinise other European leaders to see whether Merkel's austerity message, under attack last year, has gained in authority.
French President Francois Hollande, critical of belt-tightening and structural reforms, was the first to call Merkel to offer congratulations. They "expressed their willingness ... to continue their close co-operation to meet the challenges of the European project", the Elysee Palace in Paris said.
Olaf Boehnke of the thinktank the European Council on Foreign Relations said Merkel's strength was that she did not show ambitions for grand or dangerous ideas, using instead logic and time to advance her plans.
Allocation of seats
Chancellor Angela Merkel's bloc won the largest number of Parliament seats but fell short of a majority, which would have been 316 of the 630 seats.
Seats were allocated as follows:
311 Christian Democratic Union/Christian Social Union
192 Social Democratic Party
64 The Left
63 Green Party
None of the other parties won enough votes to claim seats.
