Split Social Democrats could sink Merkel's coalition plans

By Madeline Chambers and Tom KäckenhoffBERLIN/DUESSELDORF (Reuters) - The leader of Germany's Social Democrats (SPD) tried on Tuesday to quash a grassroots rebellion against a blueprint agreed with Angela Merkel's conservatives to enter formal talks on a "grand coalition" which could scupper her plan for a fourth term.After five days of talks, SPD leader Martin Schulz agreed a framework deal with Merkel and Bavarian conservatives on Friday.Many rank and file members accuse Schulz of selling