The United States Constitution is designed in such a way that the three branches of government hold nearly equal powers. While it may be argued that one branch is more powerful than the other two, each branch may offer checks and balances on the other branches. The Constitution enumerates these checks and balances as well as the limits of the branches and the members therein. Moreover, the branches cannot intersect, and therefore, they cannot directly influence each other by sharing members.
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been increased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office. (section 6, clause 2)
An example of a Congressional (Legislative Branch) check is the power to override a presidential veto with a two-thirds majority vote in both the House and the Senate. An example of a Supreme Court (Judicial Branch) check is the power to declare a law unconstitutional. Finally, an example of a Presidential (Executive Branch) check is the power to declare a national state of emergency and override the need for congressional approval for a major act.
No one branch of government is more powerful than another, at least in theory. The Constitution set up a system of checks and balances to keep any branch from having too much power. Congress, the legislative branch, can pass laws, but the President can veto the law. Congress could override the veto with a 3/4's vote. The Supreme Court, the judicial branch, could declare a law unconstitutional or a veto unconstitutional. The President, the chief of the executive branch, can promote programs, but it takes Congress to enact a law. The President can appoint people to various positions in government, but Congress must approve the appointments. The President is the head of the armed services, but Congress passes a budget to fund these programs. Only the president can declare war with the backing of Congress. This is only a short synopsis of the checks and balances. There are many more examples.