Should Berlusconi lose, he must tender his resignation to President Giorgio Napolitano, who would then try to determine who might be able to cobble together enough support to form a new government.

It now appears unlikely that Berlusconi would be asked to do so, because his only chance of forming a new majority would be with Fini's help, and Berlusconi has already turned his back on the possibility of Fini playing an increased role.

Other possibilities to form a government include Fini and Pier Ferdinando Casini, another ally-turned critic of Berlusconi. But it is not clear if Berlusconi would allow his supporters to back either of his main critics.

Should nobody be able to form a majority, a non-partisan technical government would take over in order to push through electoral and fiscal reforms, or Napolitano would call for new elections.

But the most intriguing possible outcome is that with Berlusconi losing the vote, one of his key lieutenants would succeed as prime minister. The most likely candidates are Secretary to the Council of Ministers Gianni Letta and Finance Minister Giulio Tremonti.

In theory, either would be acceptable to centrists who have become increasingly suspicious of Berlusconi, and Berlusconi himself could step aside to take a lesser role, perhaps as foreign minister, which would allow him to maintain immunity against prosecution.

One drawback is that neither Letta nor Tremonti has ever led a government or even a political party. Both are effective in their roles within the government, but it is not clear how effective a leader either would be.