BERLIN (Reuters) - A majority of Germany's Greens voters would be ready to form an alliance with Chancellor Angela Merkel's conservatives if the chance arises after September's election, a poll showed on Wednesday, raising slightly the possibility of a new coalition.
In a Forsa poll shortly before a Greens pre-election conference due this weekend, 54 percent of the party's supporters said it should be ready for a conservative-Greens coalition, so far untested at the national level.
However, the survey put Merkel's current centre-right coalition between her conservatives and the pro-business Free Democrats (FDP) on 47 percent, the third straight week it had a ruling majority.
The main opposition Social Democrats (SPD), up one point at 23 percent in the poll, are the traditional partners of the Greens and the two ruled together between 1998 and 2005.
Although most polls put the conservatives ahead with Merkel in pole position to take a third term, it is less clear who her coalition partner could be.
Merkel says she wants to rule again with the FDP but their support has tumbled since the 2009 election. The Forsa poll put them on 5 percent, just over the threshold to enter parliament.
If the conservative camp and FDP fail to win a ruling majority, Merkel may be forced to share power with the SPD in a 'grand coalition', which ruled from 2005 to 2009.
A conservative-Greens alliance is another, less likely option, but the parties have different priorities and opposition from traditional voters could make a national coalition tricky.
Guellner said things may be changing.
"Greens supporters come more from the middle classes and hardly at all from the proletariat," said Forsa chief Manfred Guellner.
"Until now, the conservatives have rejected the idea of a conservative-Greens coalition. If they change this hard line, the Greens would have no reason any more to decline an offer from the conservatives," he added.
The Greens have performed well in the last few months but they lost one point this week, falling to 14 percent.
The SPD's chancellor candidate, Peer Steinbrueck, picked up two points after a party conference earlier this month, the poll showed, but is still trailing Merkel in the personal popularity ratings with just 19 percent compared to Merkel's 57 percent.
Germany's new anti-euro Alternative fuer Deutschland party, which could steal votes mostly from Merkel's conservatives and the FDP, have failed to make a big impact in the polls so far and won only 2 percent in the Forsa survey.
(Reporting by Madeline Chambers, editing by Gareth Jones)