Finance Minister Peer Steinbrueck has increased the pressure on Kurt Beck, leader of Germany's Social Democrats (SPD), by openly criticising him for approving loose cooperation with a far-left party, reuters reported. Steinbrueck is one of three deputies to Beck and his criticism adds weight to an internal party rebellion against the SPD leader over his decision to use Left party votes to help the SPD seize power in the western state of Hesse. Beck is the presumed challenger to Chancellor Angela Merkel in next year's federal election, but polls have shown his support sliding as criticism of his stance towards the Left party, a group of ex-communists and disgruntled former SPD supporters, mounts. In an interview with the Frankfurter Rundschau newspaper, Steinbrueck said a majority of SPD voters rejected the idea of cooperating with the Left and said Beck's decision to work with them in Hesse was "not the last word" on the matter. "Such a decision must be better communicated and prepared internally," Steinbrueck said in a clear swipe at Beck, who did not consult senior party members before changing his stance on the Left -- a party he had vowed previously never to work with. Steinbrueck made clear he opposed not only active cooperation with the far-left party but also "tolerating" them by using their votes to take power in Hesse, as Beck envisions. The SPD has been outshone by Merkel and her conservatives in Berlin's awkward "grand coalition" and Beck, who took over the leadership of the party two years ago, has been struggling to slow her momentum ahead of a 2009 federal vote. Loose cooperation with the Left would help the SPD oust a major Merkel ally in Hesse, incumbent premier Roland Koch, but it is a risky move because the far-left party is seen by many within Beck's party as dangerously populist and unreliable. Beck, who is suffering from flu that led him to cancel his appointments for the coming week, has remained largely silent as members of the SPD and Merkel's conservatives have attacked him over the past week. The SPD has few attractive alternatives to Beck as challenger to Merkel next year.