BERLIN (Reuters) - The leader of Germany’s pro-business Free Democrats (FDP) poured cold water on the prospect of a national alliance with the ecologist Greens and the centre-left Social Democrats (SPD) on Sunday, saying he saw little common ground. A slide in support for Chancellor Angela Merkel’s conservative Christian Democrats (CDU) and their Bavarian CSU sister party - together known as the “Union” - ahead of a September’s parliamentary election has focused attention on various coalition scenarios. An opinion poll published on Sunday put support for the conservatives at just 25%, with the Greens closing in, on 23%. The FDP, which has been emboldened by gains in a regional vote this month, could be a kingmaker. One scenario that is becoming more likely, at least mathematically, is a Greens-led alliance with the SPD and FDP, dubbed a “traffic light” coalition after the parties’ colours. The three parties already govern together in the western state of Rhineland-Palatinate. But FDP leader Christian Lindner seemed sceptical about whether the alliance would work at federal level. “Where is the common ground in terms of economic policy?” he told ZDF television. “They want more debt and higher taxes, we want to get out of debt and tax less.” Although the FDP has criticised the ruling conservative-SPD alliance for its handling of the pandemic and what it says is a violation of civil liberties from lockdown measures, the conservatives are the most natural fit for the FDP’s pro-business policies. Lindner said he saw a lot of common ground with new CDU party chairman Armin Laschet, particularly on questions of economics and financial policy. Laschet and CSU leader Markus Soeder want to settle the issue of who will be the conservatives’ candidate for chancellor by May 23.