The honeymoon is always too short. Martin Schulz, the so-recently-celebrated chancellor candidate for the Social Democratic Party (SPD), has lost hard-earned ground to Angela Merkel in the opinion polls.

The latest poll by Germany's main public broadcaster ARD put the SPD at only 27 percent - four points down on mid-April - while Merkel's Christian Democratic Union (CDU) managed to increase its score by three points to 37 percent.

Even worse for the Social Democrats was the personal polling for the two candidates. Asked who they would vote for if the chancellor was elected directly (which he or she isn't - Bundestag members do that), 49 percent of Germans polled said Merkel, while only 36 percent said Schulz. In mid-April, the score had been much closer: 46 percent for Merkel, 40 percent for Schulz.

It feels like a haunting comedown for the center-left. The SPD enjoyed a major boost (over 10 points in some polls) when Schulz's candidacy was announced at the end of January, after party leader and then Economy Minister Sigmar Gabriel stepped aside, and it briefly had Germany's two biggest parties neck and neck.

But that sugar-high has now all but dissipated - though the center-left party is still polling at way above the 20 percent it had slumped to in some polls in January, and though it has gained over 15,000 new members this year.

Saved by Kraft?

The poll couldn't have come at a worse time for Schulz, as the SPD faces a must-win election on Sunday in Germany's most populous state: North Rhine-Westphalia, which has traditionally been a stronghold of the SPD. The government there is also currently led by SPD state premier, Hannelore Kraft.

Matthias Micus, political scientist at the Institute for Democracy Research in Göttingen, said this kind of crash was inevitable, given the circumstances. At first, he argued, Schulz's popularity was largely carried by greater confidence inside the party. "The poll ratings climb, the mood in the party base climbs with it, the party base campaigns more confidently - the multipliers are more motivated, which results in the poll ratings climbing even more," he told DW.

But there is a big but. "Apart from that, it was completely crazy the way the media and the party believed that just by naming a new candidate, without changing a single thing about the flaws and weaknesses of the Social Democrats, a situation they'd been stuck in for years, they could change the fortunes of the party," he said.

The problems with Schulz as a candidate perhaps should have been obvious, and aren't necessarily his fault. For one thing, as former president of the European Parliament, he was virtually unknown in Germany, and for another, the sudden announcement of his candidacy gave nobody in the party - least of all himself - any time to prepare him for the campaign. He was, in effect, simply superimposed on a campaign that had already been prepared without him.

Micus said the euphoria that accompanied Schulz's initial selection reminded him of the beginnings of a new political movement, rather than a new strategic move by an established political party that is, after all, still in government. "It was all great expectations of the future, without any boundaries imposed by Realpolitik ," he said. "We know that from small political movements in their foundational phases, when they have almost religious expectations of their leaders."

The fact that such a mood should have emerged in a party as large as the SPD, which has so much government experience and a well-established political apparatus, "borders on madness," said Micus. "And it can only be explained by the previous phase of deep depression and despair."

SPD paradox

Therein lies the germ of the SPD's recent malaise. It has been the junior partner in two of Merkel's three coalition governments, and governs many of Germany's 16 states, but its assertion that it has been behind much of Germany's economic success in recent years never seems to reach the electorate.

"Instead it led to the chancellor's party profiting from it," said Micus. "And then the SPD felt humiliated by public opinion - they felt not properly appreciated by people."

Schulz's brief bounce was also initially helped by his announcement that he would "correct" the neo-liberal social welfare reforms, known as "Agenda 2010," implemented in 2003 by the last SPD chancellor, Gerhard Schröder. Many have blamed these reforms for the party's loss of its working-class voter base.

But those promises, as Micus pointed out, are made by the SPD during every election campaign. "It's an old pattern," he said. "In 2005, Schröder offered social justice again - but between elections the SPD pursues business-friendly policies. They probably would've said the same with Gabriel, but that doesn't make the SPD more credible - the voters don't believe them."

Schulz's other problem is his opponent. Merkel has now honed her persona to perfection - the chancellor who stands above party politics as a unifier of the German population, and who manages government, rather than directing it to her own will. Her personal ratings suffered damage during the refugee crisis - but it was not the kind of damage the SPD could exploit. And that's mainly down to the fact that the SPD, as part of Merkel's government, still lacks clarity.