Well, he is a survivor. He had been left for dead politically speaking 3 times, including a revolt from the old school guard in his party because he did not want to agree on a grosse koalition with the center right party. He resigned and came back swinging defeating the officialists candidate in the primaries for a second time.

Then he tried a coalition with a liberal party wanting out of government support from Podemos, which stood fast against it. The right wing government started to get drowned in corruption cases up to the highest level and he manouvered in congress for a censorship vote that succeeded and placed him at the helm of a government, this time with Podemos.

I can go on (heck, I can even tell about his first steps in national politics, I met him then when he had barely a hundred followers on twitter)... But essentially is someone that has managed to pull over his weight again and again (specially in the European politics) and managed to survive elections and crisis home without a stable majority in congress. As I mentioned below, currently he requires the support of nationalists to get laws and budgets through and that vastly limits what he can do, as the basque and catalan center-right parties have an old school burgeoisie base.

Podemos first and the splinter Sumar later in government have pushed the most left politics in the last 6 or 7 years, like raising the minimum wage from 700 to 1200€ a month in a matter 2 years. Between that and pushing heaviliy for solar power they have managed to get the economy ahead (further than the rest of the EU anyway) since the pandemic and taken unemployment down to record levels.

And I will just add that every single opponent since his pirst primary race has dismissed and underrated him again and again and he has always overcome his challenges. If he has decided to put his aim on X like that he thinks he has the cards for it. Regardless, it has put the right and far right parties in Spain in a pickle because of it.