About 20 million Germans, almost one-third of the electorate in the country of 80 million, were predicted to tune in as the candidates met in a 90-minute televised debate.
With almost half the voters still undecided, Martin Schulz, a former European Parliament chief and head of the Social Democratic Party, hoped to halt a major slide in the polls.
A recent poll had shown Angela Merkel's Christian Democratic Union ahead as they began their debate on topics ranging from domestic security to foreign policy.
But Mr Schulz appeared to surprise her with a promise to push for an end to the negotiations on Turkey's accession to the European Unionif he were elected chancellor.
"If I become chancellor, I will not only cancel the agreement, but I will also call off Turkey's accession talks with the EU. We have got to the point where no German citizen can safely travel to Turkey. We have got to a point where we must end the financial and economic ties, the customs union and the accession talks. We cannot do that alone. We must talk to our European partners about it. I think the next chancellor has the duty to protect Germany by saying to Turkey that all red lines have been crossed and, therefore, this country cannot become a member of the EU -- which is actually difficult for me."
Turkish president Rejep Tayyip Erdogan launched a huge purge of state institutions after military officers tried to oust him a year ago.
More than 50,000 people have since been arrested, including more than 170 journalists and many opposition politicians, academics and activists.
Responding to Mr Schulz, Angela Merkel initially cautioned against such a move by the European Union.
She said it would be irresponsible to endanger ties with Turkey at a time when 12 German citizens are imprisoned there on political charges.
"I was never in favour of Turkey joining the EU, and now I will reflect carefully on the options. I am immediately in favour of stopping pre-accession aid, but whether we close the door or whether Turkey does, we will have to see. Above all, I now want to help the 12 people, or it is now 14, who are sitting in prison for political reasons, and, here, I see limiting economic ties as a very important point." n ...)
But after the moderators moved on and asked a question about United States president Donald Trump, Ms Merkel returned to the Turkey issue.
She appeared to agree with ending the membership talks.
"Back to Mr Erdogan, of course we need a clear position here. I have a lot of experience talking to Mr Erdogan on this issue. Secondly, we are in agreement: no pre-accession help, and the fact that Turkey should not become a member of the EU is also clear. Apart from this, I'll speak to my colleagues to see if we can reach a joint position on this so that we can end these accession talks."
Another hot topic was migration, with Martin Schulz accusing Angela Merkel of failing to find a Europe-wide solution to the crisis.
But she defended her policies, saying they were absolutely right.
After the debate, Christian Democrats secretary general Peter Tauber praised her performance in the debate.
"Angela Merkel was just as people know her: calm, circumspect, level-headed, as a chancellor should be rather than just aiming for quick election points, and I thought that was very good."
But German justice minister Heiko Maas, of the Social Democrats, says Martin Schulz was clear on both domestic and foreign policy.
"Above all, I thought Martin Schulz's closing remarks were very convincing. You can see that he is a great European, and that is important. We won't solve today's big challenges alone, we need a strong Europe, and, for that, he is the best person you could imagine."
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