[1] As a high-profile Jewish Social Democrat, he was subject to frequent rhetorical attacks particularly by the Nazis, including a death threat by Wilhelm Frick in 1929.

During his time in the city he spoke frequently about a broad number of topics in connection with his political work, including elections, foreign affairs, economics, and history.

[2] He was an able leader of the largely moderate, reform-oriented parliamentary group and a close supporter of the party's likeminded Minister-President Otto Braun.

He also enjoyed an effective collaboration with the Catholic Centre and its parliamentary leader, Joseph Hess; together they ensured the stability of the government and navigated numerous conflicts.

Heilmann was proud of the reform and democratisation of state administration which he believed had transformed the Prussian police into a reliable ally of the republic.

Heilmann was appointed editor of the SPD weekly Das freie Wort in October 1929 and commentated regularly until the Nazi seizure of power.

In 1928 he sharply attacked the republic's enemies both on left and right; despite his reformist tendencies, he emphasised: "We must never make the mistake of thinking that we can overcome this backwardness [reaction and authoritarianism] with parliament alone."

He polemicised regularly against the Communist Party of Germany and in 1930 condemned celebrations organised by the SPD's youth wing to commemorate Karl Liebknecht and Rosa Luxemburg.

The defensive strategy advocated by Heilmann and like-minded Social Democrats prevented Hitler from taking power throughout 1932, though it could not save the Prussian government, which was deposed by Reich decree on in July.

After the November 1932 German federal election in which the Nazis lost ground for the first time, Heilmann wrote: "Today, no normal person can believe in the Hitler dictatorship anymore."

On 26 June, four days after the SPD was banned, Heilmann was arrested by the Gestapo at Café Josty and sent to Columbia concentration camp before being transferred to the police prison on Alexanderplatz.

He was mentioned only briefly or not at all in memoirs by Otto Braun, Carl Severing, Albert Grzesinski, Friedrich Stämpfer, Heinrich Brüning, and Arnold Brecht.

[2] Heilmann was first highlighted in a major publication by Hagen Schulze in his 1977 biography of Otto Braun for his role in the success of the Prussian coalition.

Wolfgang Röll, an employee at the Buchenwald Memorial, dedicated several pages to Heilmann in his work discussing Social Democratic prisoners of the camp.