However, in the election campaign for the Dutch general elections of 2006 the PvdA – for the second time campaigning under his leadership – didn't manage to maintain this position as biggest party. Some considered pension reforms proposed by him to embattle the consequences of an aging population as a reason for this decline in popularity. Others emphasized a declining confidence in Bos among parts of the electorate that perceived him as "unreliable", as reason for this. Bos lost eight seats in the elections, which saw large gains for the main rival of Labour on the left side of the political spectrum, the more radical Socialist Party. Nonetheless, after these elections (and this time successful coalition talks with Prime Minister Balkenende), Bos became Deputy Prime Minister and Minister of Finance in the Fourth Balkenende cabinet, thereby breaking a campaign promise to only join a cabinet when he could hold the office of Prime Minister.