ROME (Reuters) - Italy's center left is still in the lead a month before elections but will fall short of a majority in the Senate, a result which would make stable government more difficult, according to a poll on Friday.
The center-left's lead could be eroded by its links to the troubled Banca Monte dei Paschi di Siena, enveloped by a widening scandal over derivatives losses.
Center-left leader Pier Luigi Bersani's Democratic Party (PD) has long had ties to the Tuscan lender, the world's oldest bank.
The PD and its hard-left ally, the Ecology and Freedom (SEL) party, would fall short of a majority in the Senate if the vote were held today, a Piepoli Institute poll in La Stampa newspaper said.
That means Bersani's coalition would need the support of the centrist group led by outgoing Prime Minister Mario Monti in order to control the Senate.
Monti on Friday said the PD would be affected by the banking scandal because of its links to the bank, joining opponents of the left who are seizing on the crisis in an already bitter campaign.
Under Italy's electoral law, the bloc with the most votes automatically gets a majority in the Chamber of Deputies, but in the Senate seats are awarded by region.
A split result would make it difficult for the government to pass legislation in a bi-cameral system where laws have to go through both houses.
The La Stampa poll showed the center-left winning a majority in the lower house, but former Prime Minister Silvio Berlusconi's center-right taking the key regions of Sicily, Lombardy and Veneto in races for the Senate.
Lombardy has more seats in the 315-member Senate than any other region so is one of the keys to the control of the upper house. The center-left's lead was less than five points in a further three regions, the survey said.
A separate poll by Demos and Pi for the left-leaning newspaper La Repubblica gave the center left 38.1 percent, followed by Berlusconi's center right with 25.8 percent and Monti's centrist coalition with 16.2 percent.
A third poll, conducted by SWG, showed the center-left with a thinner lead of 34.1 percent compared to 26.6 percent for the center-right and 12.8 for Monti's centrist coalition.
(Reporting by Naomi O'Leary; Editing by Jon Boyle)