German officials will elect the country's head of state on Sunday, with outgoing Foreign Minister Frank-Walter Steinmeier widely expected to win.
After years in the shadow of Merkel, the Social Democrats are smelling blood as the chancellor faces deep divisions within her own conservative camp, and the rise of the hard-right populist party Alternative for Germany (AfD) after opening German borders to a million asylum-seekers since 2015.
The vote is due at 12:00 local time (CET).
Mr Steinmeier, Germany's former foreign minister, strongly criticised Donald Trump during the USA election campaign.
The German president has little executive power, but is considered an important moral authority and symbol of the country as its host for visiting dignitaries.
Steinmeier has always been one of Germany's most popular politicians.
He has always been one of Germany's most popular politicians.
Regarding his profile and leadership, Steinmeier stands out for his pragmatism with the media and his recent harsh statements against his USA counterpart Donald Trump, which he described as a 'hate preacher'.
In a congratulatory cable a few hours after Steinmeier was elected, Putin said he hoped for strengthening of ties with Europe's strongest economy.
His election is likely to be one of the last moments of coalition unity ahead of a parliamentary election in September in which Ms Merkel is seeking a fourth term.
A policy wonk by nature, Steinmeier served as advisor and then chief of staff to Merkel's predecessor, the SPD's Gerhard Schroeder.
Under Ms Merkel, he served twice as foreign minister - from 2005 to 2009 and again from 2013 until this year, with a stint as opposition leader in between.