Marco’s greatest legislative accomplishment was the Senate’s approval of an immigration reform bill supported by a genuinely bipartisan “gang” of whom he was a prominent member. When it became clear that the bill could not be sold to extreme right-wingers in the House and the country as a whole, he repudiated it, even though, as the child of an immigrant, it was an issue that presumably had some resonance for him.
When Marco ran for President, he (correctly) noted that Donald Trump was completely unqualified, by both experience and temperament, for the job. About two months after he dropped out of the race, he endorsed Trump and offered to speak for him at the Republican Convention.
Marco clearly disliked the Senate, made a point of not being there, and stated repeatedly that he had no interest in running for re-election. About a week ago, he changed his mind, for stated reasons that, on their face, made no sense.
In short, Rubio has shown that he will sell out his ideas, his country, and his party for his own ambition. Caveat emptor.