The 44 Democrats in the Senate represent the majority (just look at the populations of the states they represent).
an interesting, but irrelevant, point. and no one is convinced that it's even accurate. first, it assumes senators represent every man, woman, and child in his state, whether a citizen or not. second, it's open to debate how you'd count the population in states which have two senators from two parties. third, majority doesn't mean much till you follow it with a prepositional phrase, and the working phrase here, according to law, tradition, and, hopefully, what you were taught in your school, is "of the senators" and not "of the people"
It uses census figures, and it's obvious how you'd count it with Senators from different parties.