The Catalan Statute was drafted by the Catalan Parliament with the Generalitat (Catalan Government) taking the lead.

The political situation in Catalunya is as follows: for over 20 years CiU (Convergència i Unió), which is a coalition of Christian Democrat Unió Democrática de Catalunya and Liberal Convergència Democrática de Catalunya held the Regional Government. The Leader was Jordi Pujol of CiU. Currently, Josep Antoni Duran i Lleida of UDC is the leader.

In 2003 Pasqual Maragall, the socialist former mayor of Barcelona (of 1992 olympic fame), led the Partit dels Socialistes de Catalunya to victory, and formed a left-wing coalition government with ERC (Esquerra Republicana de Catalunya) and IC-Verts (Iniciativa per Cataluya-Verts). This left CiU and the PP in the opposition.

Finally, the success of the tri-party coalition government in Catalunya led to PSC, ERC and IC-V to run common candidates in the 2004 senate elections, which they swept in Catalunya (the PSC would normally have run alone, in lieu of the PSOE). This means that the Catalan Senators form a Senate group separate from the PSOE, unlike in the past.

ERC was already around in the 1930's during the Second Spanish Republic, and was the political arm of Terra Lliure (free land), a short-lived Catalan Independentist terrorist group active in the late 1970's.

Zapatero gained the post of PM with the votes of ERC and IU/IC (IU is Izquierda Unida, the United Left "coalition" around the Communist Party). Between 1993 and 2000, both PSOE and PP supported themselves on the Christian Democrat nationalist parties PNV (Basque) and CiU (Catalan) as well as the Canary Coalition. This means Zapatero's government is arguably the farthest to the left that any Spanish government has been since 1936. CiU and PNV are "loyal opposition" because Zapatero is favourable to reviewing both the Basque and Catalan Statutes.

To asnwer your question, ERC has proved itself a source of embarrassments both for Maragall and Zapatero, as they are rather radical and are enjoying new prominence after huge gains in votes and seats in both the regional and national parliaments. Also, the PSC is relatively at odds with the rest of the PSOE on the issue of the Catalan Statute, and the fact that their senators are in a Catalan group rather than the PSOE group may not help.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman