MADRID — The politician whom King Felipe VI has picked to try to form Spain’s next government, after the current prime minister declined, is considered a long shot — a very long shot.

Yet this is the country that gave the world Don Quixote, and if the politician, Pedro Sánchez, the telegenic Socialist leader, succeeds, especially after facing near open rebellion in his own party, it will be considered something approaching a miracle.

If Mr. Sánchez falls short in a crucial vote on Wednesday, however, the country can expect a continuation of the uncertainty and paralysis that began with parliamentary elections in December that left no party dominant and underscored the fragmentation of Spanish politics.

And elections will most likely be held again.

“It really looks improbable that Sánchez will now make it and even if it does, it would be a very fragile deal,” said Antonio Barroso, a Spanish political analyst at Teneo Intelligence, a think tank in London.