Pablo Bustinduy is a typical Podemos MP: although holding the grand title of secretary for international relations, he travels with a rucksack, and wears jeans and a sweatshirt.

Bustinduy, 33, spent much of his 20s pursuing a career in academia in France and the US, gaining a doctoral thesis in political philosophy at the New School in New York and publishing papers on Descartes, Occupy and the indignados (outraged). He returned to Europe to join Podemos, Spain’s new leftwing party, in 2014, and now criss-crosses the continent meeting its allies and supporters – overwhelmingly young Spaniards forced to leave the country in search of work.

Reflecting on the collapse of Spain’s two main parties in the general election in December, Bustinduy said: “What happened was nothing short of revolutionary. Because even with an electoral system that promotes bipartisanship, we have this completely new landscape.”

After six months of stalemate and failed coalition talks following the election, Spain finds itself at a historic juncture before the rerun on 26 June. Having come a close third last year with 20% of the vote, Podemos announced an alliance last month with United Left (IU), a leftwing coalition including the PCE, the Communist party of Spain.

They will run on a joint ticket, containing the various Podemos regional franchises, as Unidos Podemos (United We Can). In polls reflecting this new formation, the radical leftwing bloc leapfrogs the formerly dominant centre-left party PSOE into second place, giving it a realistic chance of forming a government.

As was the case in December, the election will probably produce a four-way split, with no party winning a majority. But current polling suggests that a leftwing coalition government of Podemos and PSOE could just beat Mariano Rajoy’s conservative PP – despite it finishing first – and fourth-placed Ciudadanos, with Pablo Iglesias, the secretary general of Podemos, as prime minister.

Public patience is wearing thin after six months of fruitless talks and manoeuvring. “People won’t accept any more institutional limbo,” Bustinduy said.

For Podemos, it is time for PSOE to recognise “this historical state of affairs” and support the new kids on the block. “It will depend on the results, but I think the main pressure will be on the PSOE: to choose either to form a progressive coalition government with us, or to allow, one way or the other, the right wing to stay in power and to allow austerity politics to maintain in place,” he said.

In an interview with the Spain Report, the Spanish polling expert Kiko Llaneras said Unidos Podemos “fixes the handicap” that the left has always had in the complex Spanish electoral system: a lack of proportionality. “If Podemos and IU get exactly the same votes as at the December election, but together instead of separately, they will get 13 or 14 more seats,” he said.

Podemos supporters celebrate the election results in December. Photograph: DYDPPA/Rex/Shutterstock

IU won more than 900,000 votes in December, 3.7% of the total, but just two out of 350 seats. The latest polls have caused consternation within PSOE, with the party facing its worst result since the return of democratic elections to Spain in 1977.

Last week, speaking in the party’s traditional Andalusian heartland, the PSOE secretary general, Pedro Sánchez, blamed voter disillusionment on the coalition tussles of the past six months, the party’s tentative deal with Ciudadanos in February – “a political party alien to our beliefs” – and its failure to achieve the same with Podemos, “with whom we have more similarities”, he said.

The Andalusian PSOE leader, Susana Díaz, meanwhile, sought to draw out the contradictions in Unidos Podemos: aiming to attract moderates and those who would not normally vote, while allying with the forces of the old left, including the communists. She called it “the greatest operation of political camouflage in Spain’s recent history”.

There is discomfort in some parts of the Podemos leadership at the idea of campaigning together, with the party’s carefully honed mainstream messaging disrupted by hammer and sickle flags and anti-capitalist rhetoric.

For Bustinduy, the alliance is just a strategic grouping, not a reversion to old leftwing ways. “I think there is a certain beauty to this process of unity, but it is not the unity of the left. I’m pretty sure that 24% of our electorate [the proportion that polls suggest will vote for Unidos Podemos] didn’t wake up one day suddenly being leftists,” he said. “It’s not a return to an essence or an origin, it’s the formation of a new social alliance, with a new language and a new political culture. We are looking to the future.”