Merkel, whose centre-right Christian Democrats (CDU) have consistently led the opinion polls, is expected to become Germany's first woman chancellor and the first to have grown up in the former communist east if her alliance wins Sunday's vote.
But it is uncertain whether she will gain enough support to form the coalition she says is needed to push through deep-seated reform of Germany's sickly economy, or whether she will have to share power with Schroeder's Social Democrats.
The last opinion polls on Friday gave the CDU and their liberal Free Democrat (FDP) allies a narrow majority. Their earlier hefty lead has been cut sharply by a barnstorming campaign effort from Schroeder.
Fight till the end
Both leaders spoke at campaign rallies on Saturday as the main parties defied tradition and pledged to fight for every vote right up until the close of polling.
"It's about making sure everyone's voice counts, my own included," Schroeder said jokingly to a 20,000-strong crowd in Germany's financial capital, Frankfurt.
Merkel has pledged to raise value
-added-tax and cut wage costs
With recent surveys showing as many as 10 million voters undecided, anything may happen by the time voting ends at 6pm (1600 GMT) on Sunday, when first exit polls are due.
"At 20% plus two days before the election, the number of undecided is higher than before any other general election," Richard Hilmer, head of pollsters Infratest Dimap told the daily Die Welt on Saturday.
The levels were testament to what is at stake and the deep uncertainty many Germans feel about the future course of their country.
Five million people are out of work, the pension system is facing crisis, schools and universities are in urgent need of investment and reform, while some firms have stopped complaining about high costs and have instead shifted operations abroad.
Surveys suggest most Germans accept the system needs to change but are uncertain about how far and how fast, a dilemma facing other European countries with similar welfare states.
"We can do it better. I'm deeply convinced of that"
Schroeder's Agenda 2010 reforms have begun to change the German economy in ways unimaginable a decade ago, much to the anger of many in his party who feel the brunt of change has been borne by the less well-off.
But Merkel says he is no longer capable of governing. "We can do it better. I'm deeply convinced of that," she told a rally in the former capital, Bonn.
The stark personal contrast between the candidates has symbolised clearly the choice facing Germany.
Apparently at the end of his political career, Schroeder stood on his record of keeping Germany out of the Iraq war, appealed to Germans' deep attachment to the principle of social balance and attacked conservatives as cold friends of the rich.
The latest opinion polls gave
Merkel's CDU a narrow majority
Determined but less charismatic, Merkel has accused him of botching his reforms.
She has pledged to raise value-added-tax and cut wage costs, loosen hiring and firing rules and open up Germany's pay bargaining system in a move that would weaken the power of the unions. She says only more jobs can ensure social justice.