The head of Germany's Free Democratic Party (FDP) called party critics to order Sunday as he struggles to stop his junior coalition party dropping out of parliament in looming general elections.

Philipp Roesler, who is also vice chancellor and economy minister in the coalition government with Chancellor Angela Merkel's conservatives, urged unity among his pro-business FDP two weeks before a state vote and ahead of general elections in September.

With the party polling at below the five percent threshold for seats in parliament, Roesler told the FDP's traditional Epiphany meeting that criticism of the party chairman went with the territory.

"I have no problem with it," he told the gathering in the western city of Stuttgart.

"Credibility is always also a question of style, fairness, solidarity," Roesler said with an eye to his critics, adding he now expected a sign "that we, together, are ready to fight".

State elections are due to take place in Lower Saxony on January 20.

Development Minister Dirk Niebel told delegates that things could not continue as they were for the FDP. "It tears me up inside when I see the state of my party," he said.

But he added: "We are, as a team, not constructed yet well enough."

And he complained that it was too long to wait until May when the party's political conference is scheduled to take place for the FDP to decide on its future leadership.

Merkel, who hopes to clinch a third term in power in September, faces having to find another coalition ally if the FDP's fortunes do not improve.

But she reiterated on a visit to Wilhelmshaven in Lower Saxony on Friday that her Christian Democratic Union wanted to stay in coalition with the FDP both in the northwestern state and nationally.

The FDP peaked in the 2009 national election grabbing more than 14 percent of the vote and formed a coalition with the CDU but has seen its ratings dwindle amid internal squabbling and political strategies that have backfired.