Mr. Bosch said that E.R.C. could still offer support to Mr. Mas, but without joining his government and as part of a “plural project” toward Catalan sovereignty. He warned that this meant Mr. Mas would not only have to call the promised referendum on independence, despite Madrid’s opposition, but also stop making budget cuts in sensitive areas like health and education.

Agreeing to such fiscal red lines, however, could make it harder for Mr. Mas to meet budget deficit targets set by Madrid, as well as reduce a Catalan debt pile that is by far the largest in the country — about €42 billion, or $54.4 billion, out of a total of €140 billion for Spain’s 17 regions.

In fact, until his separatist project put him on a collision course with Mr. Rajoy, Mr. Mas had won plaudits from Madrid for imposing some of the deepest spending cuts among Spain’s regional governments.

Edward Hugh, an independent economist based in Barcelona, noted that any increased dependence by Mr. Mas on more radical separatist lawmakers in order to stay in office “may well bring the independence issue to a head much sooner than many think.”

Furthermore, tensions between Catalonia and Madrid could intensify because Mr. Rajoy may be less inclined to offer a weakened Mr. Mas the kind of fiscal concessions that would have helped defuse the separatist challenge.

“From the point of view of finding an orderly resolution to the underlying issue, the result is surely not good news,” Mr. Hugh said.

Mr. Mas called the election after failing to convince Mr. Rajoy to ease Catalonia’s federal tax burden, as part of a system that redistributes taxes from wealthy regions like Catalonia to poorer ones. The vote also followed a huge pro-independence rally in Barcelona on Sept. 11., which highlighted longstanding resentment toward Madrid in Catalonia, which has its own language and culture.