* FDP leaders put on united front, strike combative tone
* Merkel wants to renew coalition with FDP after Sept election
* Poll puts FDP below threshold to enter parliament
Germany's Free Democrats, Angela Merkel's junior coalition ally, pledged on Sunday a combative election campaign based on fiscal discipline and liberal social policies, but a poll suggested they might not even make it into the next parliament.
Published on the second day of a convention where FDP leaders vowed unity after months of squabbles, the poll showed the pro-business party dropping a percentage point to 4 percent, below the 5 percent threshold for entering the assembly.
The FDP's persistent weakness spells trouble for Merkel's conservatives, who are seeking a third term in September's election. Both Merkel and the FDP want to renew their coalition.
The FDP, which won nearly 15 percent in the last election in 2009, struck a defiant note at their gathering in Berlin.
"From now on, we are putting on the yellow-blue combat gear and we are going to fight for our convictions," campaign leader and party veteran Rainer Bruederle told delegates, referring to the FDP's traditional colours.
The FDP's leaders portrayed it as the only party that would hold Germany and the wider euro zone to the fiscal discipline needed to resolve its debt crisis. They said it would prevent the "orgy of tax increases" of a left-wing government.
They attacked proposals for a national minimum wage, called for a reduction in subsidies of renewable energy and lambasted the opposition Greens and Social Democrats (SPD) as authoritarians and "weirdos" who wanted to regulate everything and would choke Europe's biggest, most vibrant economy.
But the FDP also flagged its socially liberal outlook, urging greater rights for gay couples. The party is keen to shed its public image as a party fixated on tax cuts, not least because it has failed to deliver on this 2009 promise so far due to Germany's drive to cut debt and balance its books.
Despite months of open criticism of his leadership, the FDP re-elected Philipp Roesler, 40, an ethnic Vietnamese and former heart surgeon, as party chairman in an uncontested vote that underlined the party's desire to demonstrate unity.
Roesler, Germany's vice-chancellor, has often seemed close to losing his job for failing to boost the FDP's poll ratings, which sank as low as 2 percent in some surveys. It has been ousted from six of Germany's 16 state governments.
Roesler admitted in an often humorous speech on Saturday to having had some "really rubbish" evenings.
But a strong 9.9 percent win in a regional vote in January may have heralded a turnaround in Roesler's fortunes. His most outspoken critic, Development Minister Dirk Niebel, was ousted from the board.
"There's a real atmosphere of a new dawn," said FDP voter Julia Muenzenmaier. "And with this vote, we can also put quarrels behind us."
The FDP, ridiculed at the start of the year by one of its leading members as being like a muddled soccer team, has seized on the soccer metaphor and is presenting 67-year old Bruederle as its campaign "striker" and Roesler as its "captain".
The white-haired Bruederle is a rousing orator with a jovial manner who joined the FDP in 1973 - the year Roesler was born before being adopted by a German couple from an orphanage.
Roesler has a bookish and sometimes defensive air, although he was in exuberant mood at the conference where he handed Bruederle a symbolic yellow and blue ball.
"I am giving you this ball so you can score many goals for us, for the FDP, for this country...We will fight together in a team," he said.
Sunday's poll, published in the Bild am Sonntag newspaper, showed Merkel's conservatives on 40 percent, their SPD rivals on 27 percent and the SPD's ally, the Greens, on 15 percent.
If the FDP fails to build more support, the conservatives could end up renewing a "grand coalition" with the SPD like the one that governed Germany in Merkel's first term in 2005-09.
Failure to win any seats in the Bundestag lower house would be an ignominious fate for the FDP, who have spent more time in government in Germany than any other party since World War Two.
Despite its current poor performance, pollsters say they expect the FDP to get back into parliament because its actual support is greater than opinion polls suggest.
"A lot of people have been put off the FDP because of its public image with lots of infighting, which is a shame because it has a great and convincing programme," said Gerhard Dieterle, 63, a management consultant and longtime supporter.
"I hope this convention helps us to turn the page."