BERLIN (Reuters) - Chancellor Angela Merkel’s Bavarian ally has signaled it could back a coalition between their conservative bloc and the opposition Greens after a national election due in September.

German Chancellor and head of the Christian Democratic Union (CDU) Angela Merkel (R) and head of Christian Social Union (CSU) Horst Seehofer attend a news conference after meeting of their conservative bloc to discuss their election programme in Berlin, Germany, July 3, 2017. REUTERS/Axel Schmidt

Merkel’s bloc - comprised of her Christian Democrats (CDU) and their more right-wing Bavarian sister party, the Christian Social Union (CSU) - is expected to win the election but to need a coalition partner again to form a government.

“Of course the Greens wouldn’t be a pleasant partner but election results create coalitions,” CSU leader Horst Seehofer told German newspaper Welt am Sonntag.

The CSU and the left-leaning Greens have long been seen as particularly hostile to each other so for decades it was seen as unlikely that they would work together at the federal level.

The two parties have recently been sharply at odds on the vexed issue of migration. The CSU has urged a migrant cap of 200,000 per year while the Greens pride themselves on being migrant-friendly.

But the conservatives and Greens are already working together in several regional governments and some polls suggest a tie-up might be mathematically possible at the national level after a Sept. 24 election.

Opinion polls show the pro-business Free Democrats (FDP), a much more natural coalition partner for Merkel, returning to parliament in September after failing to clear the 5 percent threshold in the 2013 election.

Another option for Merkel is a continuation of the current ‘grand coalition’ with the main opposition center-left Social Democrats but that is usually seen as a last resort.

Seehofer said he had nothing against the SPD but a grand coalition was not the ideal solution in a democracy. A grand coalition means the opposition in parliament is left looking small.

Several senior SPD politicians called at the weekend for their party to rule out a coalition with the radical Left.

Talk of a “red-red-green” alliance of the SPD, Left and Greens had been rife earlier this year when the SPD soared in the polls after choosing Martin Schulz as their candidate to challenge Merkel.

Recent opinion polls suggest there is no longer enough support for such a left-leaning tie-up.