BY MARCUS WALKER

BERLIN—Germany’s slow-burning election campaign has finally sprung into life with less than a week to go, as controversy and shifting voter sentiment force some parties to fight for their political existence.

The leaders of Chancellor Angela Merkel’s current coalition partner, the pro-business Free Democratic Party, were left pleading for votes on Monday after a disastrous defeat in Bavarian regional elections the previous day.

The Greens, part of a center-left opposition that hopes to oust Ms. Merkel, are reeling from revelations about their past tolerance of a pro-pedophilia faction within their ranks.

Instead of hitting their core environmental message as the …