Anti-communist pastor who could turn out to be Merkel's nemesis

He is Germany's answer to Nelson Mandela and if public popularity was the deciding factor, he would be voted in as the country's new president by a huge majority today in the crucial election that will determine the future of Chancellor Angela Merkel's government. Joachim Gauck, a one-time East German dissident pastor has all the experience, probity and stature that a majority of Germans yearn to see in the person occupying the highest office in the land.

The 71-year-old is the presidential candidate of Germany's Social Democratic and Green parties with a record that few fail to admire. When he was 11, his sea captain father was shipped off to a Soviet Gulag for opposing communism; the loss marked Joachim Gauck for life. He became a Protestant pastor in the port city of Rostock and spent decades fighting the totalitarian regime and helping those who fell foul of it. He has described life under communism as a "daily insult".

In the months leading up to the fall of the Berlin Wall in 1989, he was one of the leaders of dissident political groups that fought to topple communism. In 1990 he was appointed head of reunited Germany's massive archive of Stasi files. He restored a sense of justice to millions of east Germans by giving them a chilling insight into the workings of a police state and enabling many to confront their former persecutors. "For me, freedom is one of the highest ideals – I often think that many Germans, particularly those in the west, tend to be more interested in security," he said. "But in the east we idealised freedom because we never had it."

With opinion polls that put him a clear 10 points ahead, Mr Gauck is treated like a pop star wherever he appears on his presidential campaign trail. An internet fan club has attracted more than 40,000 members. The media has also fallen in love with him: "Joachim Gauck – the better president" is how Der Spiegel magazine branded him on its front cover.

Yet despite his runaway popularity, Mr Gauck's chances of being elected as the new head of state in today's snap presidential election rest on a knife edge, for ordinary German voters will not be taking part in his election.

Under Germany's postwar constitution, presidents are elected by the Federal Assembly – a special 1,244-seat electoral body comprised of German MPs and delegates from the country's 16 federal states. And so party politics dominate the contest and, on paper at least, they appear to favour Mr Gauck's main conservative opponent, Christian Wulff.

The 51-year-old is a career politician, a member of Chancellor Angela Merkel's Christian Democratic Party since he was a schoolboy. He has served as the unremarkable prime minister of the state of Lower Saxony for the past seven years. As Germany's Die Zeit newspaper scathingly remarked last week: "He is brutally superficial". Mr Wulff answers his critics with pithy one-liners such as: "The future belongs to those who are gentle in spirit and peaceful." He claims that, as the youngest German to hold the post, he would bring a breath of fresh air to the role of president.

Publicity has been given to his wife Bettina, 37, who works as a press officer for a high-street chemist, and more specifically the tattoo on her upper arm. Commentators have remarked that if her husband is elected, she will enjoy the distinction of joining Britain's Samantha Cameron and become the European Union's second tattooed leading lady.

While bidding for the presidency, he is on record as having said that he was not enough of an "alpha male" to want to become German Chancellor. Yet within the Christian Democratic Party he is seen as a rival to Ms Merkel and as her potential successor.

Germany's presidential election is taking place three years ahead of schedule following last month's shock resignation of Horst Köhler, the former International Monetary Fund chief, who stepped down after a row over remarks he made about Germany's presence in Afghanistan.

Mr Wulff is the conservative front runner nominated by Ms Merkel's Christian Democrats and the liberal Free Democrat coalition partners. The leaders of both ruling parties are hoping that they have got their sums right and are banking on a 21-vote federal assembly majority in favour of Mr Wulff today that would propel him into Berlin's Bellevue presidential palace.

From Ms Merkel's standpoint, Mr Wulff would then no longer pose an internal threat to her as Chancellor and she would also have placated her enemies with her own party who want to see a reliable conservative politician in the job. His victory would signal that her ailing and shaky coalition government, whose popularity is at a four-year low, has regained control after the drubbing it has received during the Euro crisis and in a recent key state election.

But there are hints that Mr Wulff's once-safe federal assembly majority is disintegrating. In eastern Germany, the pro-business Free Democrat Party in several states is rebelling against the party leadership in Berlin and has indicated that it will throw its weight behind Mr Gauck instead of Mr Wulff.

Describing Mr Gauck as a "really great candidate", Peter Goetz, of the Free Democrat party in the east German state of Brandenberg, is the latest regional liberal leader to indicate that his section of the party would not be voting for the party man. Liberals in the east German state of Saxony have also indicated they will back Mr Gauck. "My conscience forbids me to vote otherwise," insisted Tino Günther a Saxon liberal who was once an anti-communist dissident.

Ms Merkel's choice of the lacklustre Mr Wulff has begun to irritate members of her party. Kurt Biedenkopf, the veteran conservative former prime minister of Saxony, has called on Ms Merkel to drop her attempts to determine who will become president and allow a "free vote" in the federal assembly. He has made no secret of his support for Mr Gauck.

There have been suggestions that Ms Merkel has misjudged the presidential race so badly with her choice of Mr Wulff that her future as Chancellor is now at risk. Political commentators point out that if her candidate fails to win today's election, it would be a devastating vote of no-confidence in her government.

"If Wulff loses, the mistrust between the governing coalition partners will be impossible to stop," is how Karl-Rudolf Korte, a leading political analyst put it. "A general election would be the tragic result of such an outcome."