Spain’s two main political parties—the ruling center-right Partido Popular (PP) and the center-left Socialist Workers Party (PSOE)—have been battered by corruption scandals and the country’s economic malaise. For the first time in more than three decades, neither is expected to come within spitting distance of a majority.

The power to get one of them to a majority will most likely lie with Ciudadanos (Citizens, or C’s), a 10-year-old political party born in the restive region of Catalonia, and its young leader, 35-year-old Albert Rivera.

In its latest poll, the Centro de Investigaciones Sociológicas (CIS), Spain’s national polling service, estimated that C’s would get 14.7% of the vote, compared to 29.1% for the PP and 25.3% for the PSOE. Another poll in El País, Spain’s largest newspaper, said that C’s would be the kingmaker in the 350-seat Parliament with 72 to 84 seats, compared with 93-100 for the PP (which now has 186) and 88-98 for the PSOE (which has 110).

“The moment is a perfect storm for Ciudadanos to have great results,” says Jordi Pérez Colomé, who profiled Rivera for the online newspaper El Español.

So, who are these guys?

C’s was launched in Barcelona in 2005 by left-leaning intellectuals who were unhappy with increasing Catalan nationalism in the region of Catalonia, of which Barcelona is the capital.

Rivera joined the platform not long after it launched. An only child whose “mixed” parents (Catalan father; mother from Andalucía) ran an appliance store in the small Catalan city of Granollers, Rivera was a competitive swimmer who’d studied law before going to work at a local bank.

With public speaking skills honed as a debate league champion, little political baggage, and the clean-cut looks of a Young Republican, Rivera was soon elected party president.

In its first years, C’s focused exclusively on Catalonia and what it saw as the political establishment’s obsession with divisive identity politics (Catalonia vs. Spain), above the economic and other issues that affected most people.

It was a tumultuous start: C’s won three seats in the regional government after a campaign that featured posters of a naked Rivera, hands over crotch, with the slogan that the party wasn’t interested in a person’s clothing or language, but in the person. Not long after, Rivera raised eyebrows again when the party almost collapsed after he engineered an ill-conceived alliance with the right-wing Libertas party for the 2009 European Parliament elections.

The chance for C’s to be something more than an anti-nationalist Catalan party arrived with the economic crisis that hit Spain in 2008. Soaring unemployment, a tidal wave of evictions, and a rash of corruption scandals soured Spaniards on the PP and the PSOE. People began to talk about the need for a “second transition,” like the one in 1975 that had brought the country to democracy after nearly 40 years of dictatorship.

“Rivera saw there was a window because there was huge disaffection with political elites, and decided to jump from regional to national arena, from being a single-issue party to a renewal party,” says Antonio Barroso, an analyst at Teneo Intelligence, a political risk consultancy.

In late 2013, Rivera’s party announced that it would expand across Spain. It was around this time that Rivera joined up with Luis Garicano, a Spanish economics professor at the London School of Economics. “Albert Rivera is a very solid politician in terms of communication and discourse. But the work done by Garicano and the people around him consolidated the base, the content, and the ‘state’ part. Without that, C’s wouldn’t be what it is,” says Jorge Galindo, editor of the politics blog Politikon.

Armed with specific policy proposals based on Garicano’s ideas, the party that emerged on the national stage was something new in Spain: a market-friendly but socially progressive party that didn’t fit the traditional left-right axis.

“At one level, it’s a non-dogmatic party: Not just for the market, or for the state. The idea is to create public policies that work. We look for success stories in other countries,” says Antonio Roldán, a former political analyst at the Eurasia Group and doctoral candidate under Garicano who is running on the C’s ticket in December.

Among the proposals there is a new employment contract that aims to reduce precarious short-term employment while at the same time making it less expensive for businesses to fire long-term employees. There are measures aimed at reducing government inefficiency and increasing transparency; a corporate tax cut; and a wage complement for low-income workers (akin to the earned income tax credit in the U.S.). Oh, and marijuana could be legalized.

While C’s is combative when fighting with nationalist parties in its home region of Catalonia, it sells its national platform as a way to basically reform everything in a non-scary way.

“Their discourse doesn’t use the terms of ‘moneyed interests’ and ‘the establishment.’ They don’t use that rhetoric because they know it would scare moderate voters,” Galindo says. “But the implication is the same: You have to change the people that are in power because they are corrupt. In that sense, C’s is, to use a paradoxical term, a moderate anti-establishment party.”

It was catnip to young professionals, and the image of Ciudadanos and Rivera as outsiders without cronies to keep happy was a boon. In last May’s municipal elections, the party got 1.5 million votes – about 6.6% of the total – and more than 1,500 local seats. (Another new party, the left-wing Podemos, also did well; Podemos-supported candidates now run Barcelona and Madrid.)

“Rivera has two great virtues: one, he’s new, at least outside of Catalonia. And two, he knows how to speak in public, at rallies, in interviews,” says Pérez Colomé of El Español.

In September, C’s won 25 seats in Catalonia’s 135-seat regional parliament, making it the largest opposition party at a time when tensions between Catalan separatist parties and the central government in Madrid have peaked. That has boosted the party’s visibility and popularity.

Of course, not everyone is a fan. PP and PSOE leaders have noted Rivera’s and the party’s lack of experience. Rivera is regularly referred to as a ‘falangist’ in social media because of the party’s dalliance with Libertas and the right-wing past of several candidates who were chosen and then discarded during the party’s expansion. And the party’s business-friendly proposals have led to some in the press to say that C’s is little more than a tool of corporate interests.

Still, unless there is a scandal in the next few weeks, it is likely that Rivera will have a major say in who governs.

“Cs has been very clear: They will support that party that has the numbers and that accepts most of its policy initiatives,” says Barroso. “From an electoral point of view, their interest is to be seen as the kingmaker at the end of the day.”