BERLIN/FRANKFURT (Reuters) – Andrea Nahles said on Sunday she would resign as leader of Germany’s Social Democrats (SPD), raising new doubts about the durability of Chancellor Angela Merkel’s ruling coalition with the center-left party. Merkel’s Christian Democrats and the SPD both bled support in last week’s European elections as voters turned away from mainstream political parties, undermining a ruling coalition that already came close to falling apart last year. Nahles, whose SPD is a junior coalition partner in Merkel’s ruling alliance, said she would resign as party leader on Monday and step down as head of the SPD’s parliamentary group on Tuesday.