ROME (Reuters) - Former Italian Prime Minister Matteo Renzi, whose Democratic Party (PD) is shedding support in opinion polls, suffered a further setback on Wednesday when two allies said they would not contest next year s election. Renzi quit as prime minister a year ago after losing a referendum on his planned constitutional reforms. He aims to return to power at the vote due by May, but the PD has split under his leadership and his prospects seem to be dwindling. On Wednesday Giuliano Pisapia, a former mayor of Milan, announced that his small leftist party, called The Progressive Camp (CP), which had been expected to join forces with the PD at the election, was disbanding. Pisapia said it had proved  impossible to continue talks with the PD , and complained in particular about Renzi s unwillingness to push through a contested law making it easier for the children of immigrants to obtain Italian citizenship. Hours later, Foreign Minister Angelino Alfano, leader of the centrist Popular Alternative party (AP) which governs with the PD, said he would not be running at the election, throwing into doubt the future of the party he founded.  The CP and AP have less than 3 percent of the vote each, according to opinion polls, but the latest defections weaken the center-left in which a declining PD is now virtually without any allies.  The left and the PD are crumbling away,  said Renato Brunetta, lower house leader of Silvio Berlusconi s Forza Italia (Go Italy!) party which is the lynchpin of a center-right alliance that is expected to win most seats at the election. On Sunday, leftist parties which had already quit the PD joined forces under the leadership of Senate Speaker Piero Grasso to form Free and Equal, a new grouping already credited with around 6 percent of the vote and expected to grow further at the expense of the PD.     Posing a potential further headache for Renzi, a parliamentary commission on banking decided later on Wednesday to call a former top banker who a prominent journalist alleged had been put under pressure by a close ally in his government. In a book published earlier this year, the previous director of one of Italy s biggest newspapers said Renzi s reform minister Maria Elena Boschi had in 2015 asked Federico Ghizzoni, then chief executive of UniCredit, to look into buying a struggling bank where her father held a senior position. The cross-party commission investigating banking crises said in a statement it had voted unanimously to call Ghizzoni to speak. A source said his hearing would be held by Dec. 23. Boschi, who is now cabinet secretary, denies ever asking anyone to buy Banca Etruria, which was eventually rescued by the state. She said this week she was taking legal action.     Renzi s PD critics say he has dragged the traditionally left-leaning party to the right and lament what they say is his autocratic, domineering leadership style.    The party has been steadily losing support since it won over 40 percent of the vote at European elections in 2014. Surveys suggest it would now poll around 25 percent, some 3 points behind the anti-establishment 5-Star Movement. The center-right bloc is made up of Forza Italia and the anti-immigrant Northern League, each with around 15 percent, and the right-wing Brothers of Italy, on around 5 percent. While the center-right is seen winning most seats at the election, opinion polls suggest it will not win an absolute majority, making a hung parliament the most likely outcome. 