“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.