What stops one branch of government from becoming too powerful?

Why this is the answer

Two answers are accepted, checks and balances or separation of powers, and they describe two halves of one design. Separation of powers is the division itself: the Constitution separates the government's power into three branches so that no single person or group ends up with too much power. Checks and balances is what each branch can do to the others once the power is divided, which is to block, or threaten to block, their actions. Specific clauses of the Constitution build those checks in. When Congress passes a bill, the President can decline to sign it and send it back with objections, and Congress can still make it law if two thirds of each chamber vote for it again. The President can make a treaty only with the advice and consent of the Senate, and two thirds of the senators present have to agree to it. When a seat on the Supreme Court opens, the President names the new justice, and the Senate can turn that choice down. The Court, for its part, can reject a law passed by Congress. Each of these pairs a power held by one branch with another branch able to refuse it. The reasoning behind it was laid out in the Federalist Papers. Federalist No. 51, printed in the New York Packet in February 1788 and addressed to the people of that state, said the best protection against power piling up in one department was to give the people running each department the constitutional means and the personal motive to resist intrusions by the others, and it put the idea in one line: "Ambition must be made to counteract ambition." The USCIS lessons state the goal more plainly. Dividing power limits the government and keeps it from violating the rights of the people. Answers that name a document or a procedure miss what is being asked. The Bill of Rights, the Electoral College and the census each have their own job, and none of them is the principle that divides power among the branches and lets each one block the others. The question wants the principle that keeps all three in line, and either phrase on its own earns the point, so choose the one you can say most clearly.