By Erik Kirschbaum
BERLIN (Reuters) - Chancellor Angela Merkel's conservatives are worried that strong mid-summer opinion poll numbers may be overstating their strength for a September 22 election after they suffered nasty surprises twice before, their campaign manager said on Monday.
Presenting a first batch of "feel-good" posters featuring giant pictures of smiling Germans that will blanket the country, campaign manager Hermann Groehe said he expects the race to tighten in the final seven weeks as it did in 2005 and 2009.
"The experience of the past serves as a warning to say that good opinion poll numbers 50 days before the vote aren't a guarantee for a good election result," Groehe told a news conference. Merkel is out of Germany on a three-week holiday.
"It's a nice tailwind but we've got to guard against arrogance," said Groehe, whose conservatives are polling around 40 percent, some 15 points ahead of the center-left Social Democrats (SPD).
Merkel's conservatives and their Free Democrat (FDP) partners would win a combined 45 percent, according to an Emnid poll published on Sunday.
The SPD and their Greens allies would win 38 percent, while the "Linke" or Left, a far-left partner-less opposition party, are on track to win 8 percent. Thus neither center-right nor the center-left would have a majority.
Groehe, general secretary for the Christian Democrats (CDU) who is managing the 20-million euro campaign efforts, said earlier on Monday he believes the SPD and Greens would break pre-election vows and form a coalition with the Left party.
"The denials are becoming more half-hearted," Groehe said, referring to the SPD and Greens promise not to get into bed with the Left at the national level due to its Communist past even though there have been left-left alliances in German states.
Campaign posters play an important role in German campaigns and all parties place them -- some larger than a small car -- on street corners, lamp posts, billboards and along boulevards across the country.
The CDU posters featured ordinary Germans smiling into the camera or at each other alongside vague-sounding slogans such as: "Good work and new ideas", "Solid financing is important because we're thinking about tomorrow," and "Growth needs foresight and a stable euro."
While Merkel's lead looks comfortable now, her party has a history of underperforming on election day.
In mid-summer 2005 the conservatives were polling 44 percent but fell to 35.2 percent in the September election. In 2009, they were at 37 percent in mid-summer polls but fell to 33.8 percent in the election.
"These posters are a call to our own voters: 'All hands on deck, the election is coming soon'," Groehe said.
(Reporting By Erik Kirschbaum; Editing by Noah Barkin)