Gauck, a former pastor and civil rights activist in East Germany, rankled many members of the The Left during his stint heading the archive sifting through the files of the country's dreaded secret police the Stasi.
The Left party has officially rejected supporting Gauck, saying they had fundamental differences with his views of East Germany's past and will vote for their own candidate Luc Jochimsen.
He was nominated for the presidency by the centre-left Social Democratic Party and the Greens to run against conservative Christian Democrat and Lower Saxony premier Christian Wulff.
Though Wulff is widely expected to be voted into the ceremonial post following the surprise resignation of Horst Köhler last month, independent Gauck has garnered more support from the public and even some politicians from Merkel's coalition parties have voiced their preference for the opposition's candidate.
But the public doesn't choose the president. Instead the Federal Assembly comprising members of parliament and representatives of the 16 German states votes on Wednesday. Though he has vehemently distanced himself from the party on several occasions, Gauck stands no chance of election without The Left's support.
The Left party parliamentary group leader Gregor Gysi has said the party would wait until after Tuesday's meeting to decide if they would back Gauck or not. Should no candidate receive a majority in the Federal Assembly in two votes, the president is elected by a plurality during a third.
Were The Left's Jochimsen to bow before the third vote, Gauck would have a better chance of beating Wulff, who rejected an invitation to meet with the socialist party.
Green party parliamentary group leader Jürgen Trittin told broadcaster ZDF he expects the vote to be “exciting,” insisting that Gauck was worthy of conservative votes.
But CDU parliamentary group leader Volker Kauder said Wulff would be elected.
“He is the candidate of the coalition, which has a majority in the Federal Assembly,” he told ZDF.
But there is a small chance the secret ballot could produce an upset. There are a total of 1,244 votes, with 622 votes for the parliament and an equal amount for state representatives. Angela Merkel's centre-right coalition has 644 votes, just 21 more than the absolute majority of 623.
“There is a certain amount of tension, it has often been so with presidential votes,” former Thuringia premier and Christian Democrat Bernhard Vogel told broadcaster RBB.