Merkel's party keeps lead, coalition ally down: poll

German Chancellor Angela Merkel gestures during a news conference after talks with Greek Prime Minister Antonis Samaras at the Chancellery in Berlin, August 24, 2012.

BERLIN (Reuters) - Support for Chancellor Angela Merkel's conservatives has nudged higher, an opinion poll showed on Sunday, but the weakness of two smaller parties highlights the trouble she may face building another center-right coalition after Germany's 2013 election.

The Emnid poll conducted for the Bild on Sunday newspaper showed the conservatives up one percentage point at 36 percent but their junior coalition partner, the pro-business Free Democrats (FDP), down one point at 4 percent, below the 5 percent threshold for entering parliament.

Merkel may also be vulnerable to a slowing economy as the euro zone debt crisis drags on, though opinion polls show steady voter support for her handling of the crisis and her insistence on heavily indebted states implementing tough reforms.

The Pirates, an upstart party which has won seats in regional elections over the past year, also lost one percentage point to 7 percent, the poll showed. The party, which wants broader Internet freedom, peaked at 12 percent in April.

This is also bad news for Merkel because the Pirates had been drawing away support from her main center-left rivals. A Merkel adviser told Reuters earlier this year that a collapse of support for the Pirates was one of her biggest worries.

Sunday's poll showed the main opposition center-left Social Democrats at 28 percent, down one percentage point, and the Greens, their preferred partner in a future coalition, up one percentage point at 14 percent.

If this tally were repeated in next year's national election and the FDP failed to win seats in the Bundestag lower house, Merkel would most likely end up forging a 'grand coalition' with the SPD as happened in 2005-09.

The Emnid poll was conducted between August 16 and 22 and canvassed the views of 2,792 people.

(Reporting by Gareth Jones, editing by Tim Pearce)