Mr. Steinbrück, 66, has said that he will not be part of another grand coalition. But it is increasingly clear that he would like to be part of negotiating whatever program the two big parties can thrash out.

The uncertainty of the outcome means that the European Union, which has been holding its breath as the German vote nears, is likely to wait a few more months before learning where Berlin would like to guide the troubled Continent.

Mr. Steinbrück, a bright but sometimes blustering politician, was regarded as a strong alternative to Ms. Merkel, 59, when he declared his candidacy a year ago. Almost immediately, however, he started to stumble, first over disclosures that he had received $1.7 million for speeches and writings, then over whether politicians were paid enough, and next over a suggestion that he would never buy cheap wine.

His party, which is still divided over a legacy of labor and welfare overhauls that it initiated when last in power under Ms. Merkel’s predecessor, Gerhard Schröder, was slow to get its campaign rolling. Even as late as July, Mr. Steinbrück seemed a somewhat reluctant slogger on the trail.

Since his lone television debate with Ms. Merkel on Sept. 1, from which he emerged looking as if he could be chancellor, Mr. Steinbrück has taken to campaigning with zest. In a format that is unusual for German election campaigns, which tend to be scripted and repetitive, he has hit the road with a strategy called “plain talk,” in which citizens sit around him as he fields their written questions — filtered through a moderator but not obviously prepared.

Mr. Steinbrück, who once governed Germany’s most populous state, North Rhine-Westphalia, has one of the readier wits on the campaign trail. “I’ll start off answering your questions, and then I think it’s a 120-minute election speech, and I will read it to make it even more boring,” he said as he took the stage one evening in Munich last week, with barely 1,500 supporters shivering through his presentation.