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.