BERLIN (Reuters) - Some senior members of Chancellor Angela Merkel s conservative bloc and potential coalition partners have backed the idea of an immigration law as a possible way to help solve Germany s migrant policy jigsaw.  Merkel, who won a fourth term in a Sept. 24 election albeit with heavy losses to the anti-immigrant far right, is considering a three-way coalition with the pro-business Free Democrats (FDP) and environmentalist Greens - a tie-up as yet untested at the national level in Germany. Among their various differences, migration is shaping up as a major sticking point due to the insistence of the conservatives  Bavarian sister party, the Christian Social Union (CSU), on a refugee cap after almost one million migrants poured into the country in 2015, with tens of thousands more since. A formal cap is rejected by the other parties - as well as Merkel. President Frank-Walter Steinmeier brought the issue to the fore on Tuesday, saying in a speech that German policy must be able to differentiate between political refugees and economic migrants so as to  define legal access to Germany . Greens co-leader Cem Ozdemir, who has long argued for an immigration law, welcomed the intervention of the president, a center-left Social Democrat.  That would be a wise step,  Ozdemir told Deutschlandfunk radio on Wednesday, saying that implementing the idea would be an urgent priority for the next government. Steinmeier s foray into domestic politics was unusual for the presidency, a largely ceremonial post though with sometimes strong moral influence, but at least generated some convergence on the immigrant issue among potential coalition allies. Armin Laschet, the Christian Democratic premier of North Rhine-Westphalia, Germany s most populous state, told the Handelsblatt daily that a CDU/CSU-FDP-Greens coalition  would create an immigration law that covers the scale of immigration .      In remarks to Die Zeit weekly, Bavarian Economy Minister Ilse Aigner stuck to the CSU demand for an annual cap of 200,000 immigrants, preventing any recurrence of the 2015 crisis,  so that no backdoors stay open and so there is no lack of clarity . The other parties argue that a cap would breach the constitution, which guarantees asylum to people persecuted on political grounds.  Concerns about immigration run especially high in Bavaria, the main entry point for refugees in 2015 when close to 1 million - many fleeing war and poverty in the Middle East and Africa - cascaded into the country. The idea of an immigration law is not new. Several parties have argued for it in the past and many experts argue it could help Germany s economy as targeted, legally controlled migration could plug gaps in the labor market. The devil would be in the detail but the FDP is keen on a points system and has cited the Canadian model. There are variants in other countries, such as New Zealand and Australia. It is unclear whether an immigration law will be enough for the CSU, who risk heavy losses in a state election next year.  Senior CSU figures are still digging in their heels. Transport Minister Alexander Dobrindt told the Augsburger Allgemeine that an upper limit is necessary and is a lot more than a matter of semantics. 