With four weeks to go before presidential elections, polls in the United States show the race for the White House is virtually neck and neck. US President Barack Obama has meanwhile admitted he was "too polite" during his first debate with challenger Mitt Romney. Democrats are hoping that a strong performance in tonight's vice presidential debate could slow Romney's momentum. It pits Democratic vice-president Joe Biden against Congressman Paul Ryan, a young hero of the conservative Republican right. RFI asked Amy Greene, a political scientist what Biden will do to catch up the lost ground?