The far-right Vox party won a seat on the committee responsible for running Spain's parliament on Tuesday, raising its national profile as the Socialists struggled to form a government following an inconclusive national election last month.
The Socialists, meanwhile, again held talks with a Catalan separatist party in a bid to gain support.
Vox became the third-largest party in a fragmented parliament in the election, more than doubling its seats to 52 after campaigning on a platform of staunch nationalism and an anti-feminist and anti-immigrant stance.
Amid some chaotic scenes and scuffles in the parliamentary chamber as the new legislature was sworn in, Vox lawmaker Ignacio Gil Lazaro was elected as one of the oversight committee's four vice presidents.
The nine-strong committee decides when bills are admitted for debate and its members represent parliament overseas, giving it considerable influence.
Founded in 2013, Vox won two dozen seats in an inconclusive election in April, the first time a far-right party had won more than one seat since Spain returned to democracy in the 1970s after four decades of dictatorship.
In the November election, Spain's fourth in four years, the Spanish Socialist Workers' Party (PSOE) won most seats but fell short of a majority.
Despite reaching a coalition pact with the left-wing Unidas Podemos, it is still scrambling to drum up enough support from other parties to control the 350-seat parliament.
Hours after the oversight committee election was picked, the Socialists sat down with Esquerra Republicana de Catalunya (ERC) in its second formal conversation aimed at securing the Catalan separatist party's backing for a Socialist-led government.
Afterwards, the two parties avoided any mention of support but said in a joint statement they would meet again on Dec. 10.
Ahead of the meeting, acting Prime Minister Pedro Sanchez had sought to temper expectations that Spain could have a government in place by December.
"I don't want to put a date on it," Sanchez told reporters on Monday. "It could be December 12, December 20 or January 8.
I don't know. But what's important to highlight is that Spain needs a government as soon as possible."
Prior to Tuesday's vote on the oversight committee ballot, a scuffle broke out between a Vox member and a deputy from centre-right Ciudadanos. Socialist party spokeswoman Adriana Lastra twisted her ankle and had to receive medical attention.
After the vote, in which Socialist Meritxell Batet was elected committee president, Catalan separatist politicians used the swearing-in process as a platform to demand freedom for jailed Catalan separatist leaders.
They also refused to take the traditional oath to uphold Spain's constitution, instead pledging to campaign for an independent Catalan republic.