MUNICH, April 24 (Reuters) - Bavarian state premier Horst Seehofer said on Monday he would stay on as leader of the Christian Social Union (CSU) to fight what he expects to be a "very difficult" national election in September and a state poll next year.
As chairman of the CSU, the Bavarian sister party to Angela Merkel's Christian Democrats (CDU), Seehofer is nominally the chancellor's ally, but he has been a thorn in her side for much of the last two years, criticising her open-door refugee policy.
The 67-year-old had indicated in recent years that he would not lead the conservative CSU into a state election in Bavaria in autumn 2018, but senior CSU officials recently urged him to stay on in his twin roles as party leader and state premier.
"The passion has not deserted me. On the contrary, it is still there in full," he told reporters in Munich, announcing he would lead the CSU into the 2018 state vote after all.
Seehofer said he believed the conservatives had a great chance in this year's federal election, when Merkel is seeking a fourth term in office, but added: "We are all aware that this will be a very, very difficult path."
Merkel has refused demands by Seehofer for a cap on migrants but the conservative allies have drawn closer as the election approaches and amid a revival in support for the centre-left Social Democrats.
Opinion polls show the CDU/CSU alliance - the so-called 'Union' - leading the Social Democrats by around 5 percentage points ahead of the Sept. 24 federal election.
Reporting by Joern Poltz and Paul Carrel; Editing by Catherine Evans