Barely three months ago, Schulz, the new head of the Social Democratic Party (SPD), seemed to be the man most likely to topple Merkel, who is running for a fourth term.
The 61-year-old's decision to take the helm of the SPD in late January had jolted the party to life, with some opinion surveys recording a then 10-percentage point jump and some polls in March even putting it ahead of Merkel's conservative bloc. Schulz's popularity soared close to 50 percent, overtaking the chancellor's 38 percent.
But three months on, the trend has reversed.
The SPD has suffered heavy losses in three straight state elections while Merkel's Christian Democratic Union (CDU) party has scored clear wins.
Nationwide, the SPD is now trailing around 10 percentage points behind Merkel's CDU and Bavarian allies CSU.
A poll published this month by national broadcaster ARD gives Schulz just a 36 percent approval rating, compared with 64 percent for Merkel.
The SPD's freefall has sparked questions about whether Merkel has already won the election even though campaigning has not yet begun in earnest.
Polls even suggest Merkel could secure a win big enough to form a coalition with the far smaller liberal party FDP, and knock its current partner SPD out of the government.
For analysts, the suddenly waning support for Schulz's SPD boils down to the government's success in curbing a refugee influx that saw 890,000 migrants arrive in 2015, deeply unsettling many German voters.
Schulz initially "was then presented as an alternative to Madame Merkel," said Gero Neugebauer, a political science analyst at Berlin's Free University.
"He was relatively new and had criticisms against the chancellor's immigration policy which had divided public opinion," added Neugebauer.
But migrant arrivals have tapered off, and Merkel's CDU and CSU "have won increasing trust that they have the best strategy to deal with the refugee situation," Renate Koecher from opinion research group Allensbach Institute wrote in the Frankfurter Allgemeine Zeitung.
No stranger to setbacks
Schulz has campaigned on a platform of social justice and wants higher earners to pay more taxes while offering small income families lower levies.
While his call has resonance with the working poor, Koecher noted that with Germany's solid economic outlook and record low unemployment, "it is likely to have limited impact".
SPD party rank-and-file gathering in the western city of Dortmund on Sunday to approve their election campaign programme will be looking for reassurance amid the poor poll numbers.
France's Emmanuel Macron and Britain's Jeremy Corbyn may provide inspiration after they showed that it is possible for social democrats to come from behind and pull off stunning upsets of the political establishment.
But analysts believe Schulz is unlikely to benefit from their momentum or copy their campaigns.
"The SPD has massive problems in picking one over the other. Not because they find both so good, but because both are what the SPD no longer want to be," noted the Frankfurter Allgemeine Zeitung, referring to Corbyn's hard-left platform and Macron's business-friendly policies.
The SPD positions itself in the middle, said Neugebauer, noting however that this lands it in the dilemma of how far left or right it should lean.
But Schulz is no stranger to bouncing back from setbacks.
After an injury dashed his dreams of becoming a professional footballer, Schulz sank into alcoholism before opening a bookstore and becoming an autodidact with six languages under his belt.
Without finishing high school, he also rose to become president of the European Parliament, an unusual achievement for Germany, a country obsessed with academic qualifications.
At a congress of German industry leaders this week, also attended by Merkel, Schulz signalled he was undeterred.
"Frau Merkel began her speech as chancellor of Germany and then at the end said she spoke as chairwoman of the CDU.
"I suggest that I do the opposite: I begin as SPD chairman and will speak at the end as the future chancellor of Germany," he declared.