The language in the linked letter seems straightforward as to the premise, “to ensure a full and thorough investigation of the Russian government’s efforts to interfere in the 2016 presidential election,” and it refers back to the already ongoing investigation that Director Comey had been leading.
Remember RR can (secretly) expand that mandate literally any time. I suspect he has because you will recall RR made that comment about not allowing a fishing expedition.
But it didn’t say “efforts to interfere on behalf of Donald Trump.”