Friedrich Merz, the Christian Democratic Union’s general secretary, is polling ahead of Mrs Merkel’s favourite to replace her, Annegret Kramp-Karrenbauer.
And today the 62-year-old conservative warned Brexit will have a major impact on Europe’s economy and played down EU hopes of a second referendum that could end Brexit.
Speaking to a discussion panel on Europe’s growth challenge hosted by politics and current affairs think tank Chatham House, the CDU leader candidate said the UK could rejoin the EU.
He said: “Brexit will happen, perhaps they will come back one day.”
He also warned Europe was on a “threshold” and said Germany is doing more for the success of the European Union than any other country.
Mr Merz added that despite euro being too weak for the German economy, it is still a good thing for his country because others in the bloc are not strong enough to benefit from the exchange rate.
He said: “Let’s face it, we are benefiting from the monetary policy which most of the people in this country do not want. But we benefit from it, fundamentally,”
“And this currency, which is in the meantime too weak for our economy, is still too strong for most of the others.
“We are benefiting within the European Union, within the internal market, but beside that we are benefiting in international trade – towards China, towards the U.S., towards other regions in the world – from this currency policy.”
“We have to tell the people in this country that the Germans have to contribute more than others to the success of the European Union,” he added.
“We have to do more than we are actually doing because if Europe fails – and this is a clear option, no one can deny it, Europe is really at the threshold at the moment – if Europe fails, the Germans will be those who suffer most from that.
“Europe is really at the threshold at the moment.”
According to a poll for Funke newspaper, Mr Merz is in the lead to replace Mrs Merkel with 36 percent support, ahead of Mrs Kramp-Karrenbauer who was close behind with 33 percent.
German health minister Jens Spahn, 38, has also thrown his hat into the ring to replace Mrs Merkel.
He has been an outspoken critic of Mrs Merkel but it is unclear if he has enough support to replace her.
Mrs Merkel announced last week that she would not seek re-election as leader of the CDU at the party’s conference next month.
She said that she would stay on as Chancellor of Germany, a move which has sparked fierce political debate and accusations of a U-turn after Mrs Merkel previously said the two jobs should not be done separately.
She is under pressure to resign as chancellor after a new crushing poll revealed two-thirds of Germans think she should quit.
Whoever takes over from Mrs Merkel has a huge task ahead of them to turn the party’s fortune around after it was revealed support for the CDU has plummeted to a new low.
Support for Mrs Merkel’s party now stands at just 24.5 percent, down from 31 percent a year ago and 26 percent last month, according to a weekly survey from INSA/YouGov on behalf of Bild.