Stephan Ernst is likely to spend at least 15 years in prison due to the seriousness of the crime, the judges in Frankfurt's higher regional court ruled on Thursday, while reserving the right to keep the 47-year-old in preventive detention even longer.

Ernst shot Luebcke in the head on the politician's own porch on June 2, 2019.

Luebcke, a member of Chancellor Angela Merkel's Christian Democrats (CDU) who led the local government in the central district of Kassel, is said to have been targeted due to his support for Merkel's open-border policy during the 2014-2015 refugee crisis.

The judges agreed with prosecutors that Ernst had been motivated by his own extremist views.

"He projected a hatred of foreigners onto Dr Luebcke," they said in the ruling.

Ernst's co-defendant, identified only as Markus H under German privacy laws, was handed a suspended sentence of one year and six months for violating arms laws.

The court did not find him guilty of aiding and abetting the crime.

Ernst had admitted to the killing but changed his version of events three times.

In his most recent testimony, Ernst placed part of the blame on H and said he had been with him at the scene of the crime.

H gave no response to the allegation, only speaking once in the trial to respond to a weapons expert's assessment and in doing so showing his own expansive knowledge on the subject.

Markus H's defence team had argued for their client's full acquittal.

Presiding judge Thomas Sagebiel said he had no option but to hand down the harshest sentence possible to Ernst but noted that the man's confession could lead to a milder punishment.

Sagebiel explained that the convicted killer could cooperate in a rehabilitation program for "right-wing extremists", which could influence the minimum length of the custodial term or help him to avoid preventive detention upon completing that term.

Ernst also faced a charge of attempted murder for another crime, in which an Iraqi refugee was stabbed in the back in 2016 and left seriously injured.

However, the defendant was acquitted.

Armin Laschet, the newly elected leader of Merkel's CDU, condemned the "cowardly murder" of his party colleague Luebcke in a statement.

"Today's ruling gives me courage... and is simultaneously a warning to us all: We will not allow right-wing terrorists and political firebrands to break our country," he wrote on the conservative party's website.

Australian Associated Press