How does a political party become the majority party in the US?

In the U.S. political system, the legislative branch of the government is represented by the National Congress, composed of the House of Representatives and the Senate. Members of both houses of Congress are appointed by direct democracy by the voting-eligible members of the appointee's constituency region - congressional district for Representatives of "the House," and states in the case of Senators. Every state has 2 senators, and the number of congressional districts (and thus number of representatives) is apportioned based on the population of that state. Currently any particular state may have between 1 (Wyoming and Delaware among others) and 53 (California) representatives.

Since the House and Senate are elected independently from each other, the proportion of Republican, Democrat, and Independent members may be different each. Thus determination of the "majority" party is technically made separately for the Senate and House based on which party has the most representatives in that particular section; i.e. "Senate Majority" and "House Majority." That said, general political trends tend to affect both sections of congress similarly and it tends to be the rule that the party that is the majority in one house is also the majority in the other, and thus "controls" the Congress.