And as a new party with a clean record, it has presented a potent challenge to the kind of corruption that has rotted Spain’s political establishment, tainting parties of nearly every stripe and leaving them increasingly vulnerable.

“The challenge for Rivera has been to turn what was a one-dimensional party opposed to Catalan independence into something more sophisticated,” said Juan Ignacio Torreblanca, a professor of politics at UNED, a Spanish university.

“Rivera has been trying to make his party the perfect response to the excesses of both the conservatives and Socialists who have so long dominated Spain,” he added.

Mr. Rivera recognizes the value of his middle-ground positioning, particularly when compared with Podemos, the other new-kid-on-the-block of Spanish politics, which also has a youthful leader, 37-year-old Pablo Iglesias.

“We don’t have to travel to the center because we’re already there, which is a lot more credible than the path of a party like Podemos that started from a far more extreme position,” said Mr. Rivera in an interview at his party headquarters in Barcelona.

In fact, the recent rise of Citizens in opinion polls has coincided with the decline of Podemos, which made strong gains in municipal elections last May and was seen as the main challenger to Spain’s established parties.

Since then, however, Mr. Iglesias has struggled to shift his party toward a more centrist electorate without losing the support of the far-left faction that founded it in 2014, after leading street protests against the austerity policies of the Rajoy government during Spain’s recession and banking crisis.

As Greece’s own economic crisis has dragged on, however, Podemos has recently been tainted by its unflinching support for Syriza, its Greek counterpart, as a role model of left-wing European economic management. In September, Podemos suffered another blow in the Catalan regional elections, when the party fared poorly after trying to stay on the sidelines of the secessionist conflict there.

“Having an ambivalent stance on Catalan independence has really hurt Podemos,” said Federico Santi, political risk analyst at Eurasia Group, a think tank based in London. “They’ve not really managed the step from protest party and critic of the main parties to one that can build its own platform.”

In contrast, in Catalonia’s recent elections, Citizens eclipsed Mr. Rajoy’s Popular Party, which has been locked in an increasingly contentious battle with Catalan leaders over their independence drive, as a more credible political force opposed to secession.

“Most politicians have tried to gain personally and politically from the Catalan issue, but Rivera has always been driven by absolute conviction,” said Josep Borrell, a Catalan professor of economics and former leader of the Spanish Socialist Party. “He was really the only Catalan politician prepared to denounce the mythology of the Catalan nation from Day 1.”

But when it comes to discussing a possible political deal after the Dec. 20 national elections, Mr. Rivera is a lot less forthcoming.

Given his party’s place in the center, it is positioned to pivot left or right, and could thus be the crucial element in any new coalition government.

Mr. Rivera, however, insisted in the interview that he would not jeopardize the long-term development of Citizens by joining a new government that is not committed to its reform agenda.

For instance, Mr. Rivera said he studied the trajectory of the Liberal Democrats in Britain, which joined a coalition conservative government led by Prime Minister David Cameron in 2010, but then slumped in elections in May, as British voters punished them for abandoning earlier pledges.

“We know what happened in England and that to enter in a government that doesn’t believe in your changes can lead you to a situation of incoherence and disappointment,” Mr. Rivera said. “We won’t get married with a party that doesn’t believe in change and in opening a new chapter in politics.”

Mr. Rajoy’s Popular Party has kept a narrow lead in polls, but insufficient to keep its parliamentary majority.

The Socialists, Citizens and Podemos are expected to each get more than 10 percent of the vote.

Podemos, however, has recently been outpaced by its main rivals. Citizens, in particular, has cut into Podemos’s support by similarly capitalizing on Spaniards’ increasing disgust with the institutionalized corruption within Spain’s two-party system.

“We don’t pretend to change the human condition and say that there should no longer be thieves, but we just want to make sure they go to prison rather than continue to sit in their office,” Mr. Rivera said. “We want to stop impunity.”

This month, Mr. Rivera proposed an overhaul of Spain’s judiciary, to help depoliticize the institution, as well as another one to convert the Senate into a more efficient, German-style upper chamber of Parliament.

“Podemos answered the question, ‘What went wrong?’ but we’re here to answer the question of ‘What to do now?’ ” said Mr. Rivera. “That’s why we are finishing this campaign stronger than ever and nearing victory.”

But even if the party has also made several specific proposals on taxation and other issues, Citizens remains something of a chameleon, flexible enough to lend its support to a conservative regional government in Madrid but also a Socialist one in Andalusia.

Citizens is hard to define, agreed Mr. Torreblanca, the politics professor, but could perhaps be called “liberal in economics, liberal on civil rights but very far to the right when it comes to defending Spanish identity.”

As part of its electoral program, Citizens is also taking aim at separatists in the Basque region, by calling for the removal of longstanding Basque tax benefits granted by Madrid.