By Christian Fraser
BBC News, Rome
Romano Prodi's fragile government faces another nail-biting vote on the Italian peacekeeping mission in Afghanistan.
Romano Prodi has only a razor-thin majority in the Senate
The Senate has to agree the re-financing for the mission.
But the parties on the far-left of Mr Prodi's alliance want the 1,800 troops currently deployed in Afghanistan to come home.
It is barely a month since Mr Prodi lost a Senate vote on foreign policy and another defeat could spell the end for the prime minister.
'Smelling an opportunity'
In previous votes on Afghanistan, the opposition forces led by Silvio Berlusconi have voted with Mr Prodi in solidarity with the troops serving abroad.
But in recent days, Mr Berlusconi has expressed his concern at the way in which the government negotiated the release of an Italian journalist kidnapped in Afghanistan by the Taleban, a deal in which five Taleban prisoners were released in exchange.
The opposition has also criticised Mr Prodi for refusing to consider changes to the rules of engagement for Italian troops.
Mr Berlusconi says this is necessary so they can defend themselves properly.
This means that on Tuesday, Mr Prodi must rely on his own coalition and a handful of lifetime senators.
The prime minister has said any senators voting against the Afghan mission will be trampling national dignity.
But the opposition can no doubt smell an opportunity, and there will be many queuing up to demand his resignation if the vote fails to muster a convincing majority.
In the end, Mr Prodi will probably squeeze home, thanks to the support of the smaller Christian Democrat Party in the opposition.
But the vote will be a sign of whether the prime minister can trust in his own coalition and whether the guarantees he received the last time the government fell will actually count.