Nobody can govern in Yemen without support from the tribes and on Saturday influential figures from the country's two main tribal groupings – the Hashid and the Bakil – abandoned President Salih.

"I announce my resignation from the General People's Congress [the ruling party] in protest at the attacks on peaceful demonstrators in Sanaa, Tai'zz and Aden," Hashid tribal chief Sheikh Hussein bin Abdullah al-Ahmar told a huge gathering in Amran province, north of Sanaa.

The Yemen Post says his announcement was "warmly received by a large crowd of tribesmen", including members of Yemen's second largest tribal group, the Bakil.

Mohammad Abdel Illah al-Qadi, a prominent leader of the Sanhan (Salih's own branch of the Hashid) also announced he is leaving the party.

Yemen's tribes are not monolithic blocs, as Gregg Carlstrom points out in an article for al-Jazeera, and Salih still has supporters among them (partly as a result of bribing them with money and cars). Even so, the latest desertions are an important sign of which way the wind is blowing.

A statement from the General People's Congress on Sunday played down the problem. It said "the resignation of some members of the GPC at this timing reveals the reality of the opportunists" and described the departures as "like an operation of purification of the GPC from the parasites that were unable to effect any development inside the organisation". It added that "the GPC has in its ranks millions of loyal and sincere members who have firm stands and work sincerely for siding with issues of the homeland and the people".

All this will come to a head with the parliamentary elections scheduled for April 27, which are likely to be conducted in the midst of unprecedented turmoil – assuming they do go ahead. They have already been postponed for by two years and to postpone them again would be extremely dangerous for Salih politically, as would rigging them to ensure another GPC victory. 

Beyond April 27, the shape of events in Yemen is anyone's guess but Salih's survival prospects are clearly fading.

Posted by Brian Whitaker, 28 Feb 2011