Who are they?
The CSU was founded in 1945 as the Bavarian sister party of the CDU, and has governed in Bavaria every year since 1949 apart from 1950 to 1953. It now governs in coalition with the regional Bavarian branch of the Free Democrats (FDP), but act as one with the CDU at the federal level.
The two parties are collectively called 'The Union' within the Bundestag but the CSU is seen as more rightwing than its partner.
What's their strategy?
The CSU's election platform is based around their traditional conservative values, stressing the protection of the traditional family. It has massive support in Bavaria holding a 29-point lead over their leftwing SPD rivals in the opinion polls.
In the Bavarian state election on September 15th, the party will be trying to gain a ruling majority meaning it will not have to govern with the economically liberal FDP.
The party is also trying to throw its weight around within the Union in the build up to the national elections on September 22nd. Seehofer wants to introduce a charge for foreigners to use Bavaria’s autobahns, something which he thinks will prove popular with Bavaria’s conservative base. The autobahn charge has been dismissed by many as mere electioneering, but the CSU insist on its seriousness and will fight EU equality laws to institute it.
Who's their leader?
Horst Seehofer, 64, has been the CSU leader and Ministerpräsident (head of regional government) of Bavaria since 2008. Before that he had served as health minister and from 2005 to 2008 as minister for nutrition, agriculture and consumer protection.
Why do they matter?
The CSU operates as a bloc with the more mainstream CDU, but the attempt to please their conservative constituents with populist policies such as the foreigners' autobahn tax could drive a wedge between Seehofer's party and more moderate bedfellows.
If the CSU are as successful in the election as polls suggest, their strong conservative agenda could well have an increased influence on any coalition led by Merkel's CDU thus shifting the government formed post September 22nd to the right.
READ MORE: Your guide - The Christian Democratic Party (CDU)