* Opinion polls show SPD loses big lead in Lower Saxony * Conservatives could pull off surprise win in state * Steinbrueck bungling campaign to oust Merkel in September By Erik Kirschbaum BERLIN, Jan 11 (Reuters) - Gaffe-prone German opposition leader Peer Steinbrueck may cost his Social Democrats what was once seen as a certain victory in a regional vote this month and a potential win over Chancellor Angela Merkel's coalition in September, a poll showed on Friday.