A prominent German pollster said Wednesday that the "chaotic" leadership of US President Donald Trump has caused an electoral decline for Germany's right-wing Alternative for Germany (AfD) party.

According to research carried out by Forsa on behalf of German magazine Stern and broadcaster RTL, the AfD is now polling at 8 per cent - a seven-month low and 4 percentage points lower than when Trump was inaugurated on January 20.

"The chaotic leadership of US President Donald Trump, which was previously celebrated, is now causing concern in the face of crises all over the world," Forsa chief Manfred Guellner said in a statement.

The Forsa poll released Wednesday shows Chancellor Angela Merkel's conservative Christian Democrats (CDU) garnering 34 per cent of the vote, with the centre-left Social Democrats (SPD) - the junior partners in her coalition - coming in at 31 per cent.

The SPD gained electoral momentum in January after former European Parliament chief Martin Schulz took over the party leadership with the intention of challenging Merkel in the September election.

"The few moderate AfD supporters have migrated to the SPD, because its chancellor candidate Martin Schulz is considered capable of replacing Angela Merkel, whom they hate so much," Guellner said.