How does a political party win the senate? Please explain.?

Americans go to the polls and vote for the candidate of their choice.

If more Republicans are in the Senate than Democrats then the Republicans become the majority party. If we elect more Democrats then Republicans then the Democrats become the majority party.

The party with the most Senators rules.

By holding the majority of the seats in the senate.
There are 100 senate seats.
So the majority is .....

Don't worry dear.
You'll learn this when you get to second grade.

Their party gets the most votes for the most open seats. It is really quite simple. 51 seats is a majority.

There are a total of 100 Senators in the United States(2 for each state).
If either party has 51 Senators, they can control the Senate.
Majority rules.