The role of Congress which consists of the Senate and House is defined in Article I of the Constitution of the U.S.A.
Congress is exclusively responsible for making law in the U.S.:
All legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives. - Article. I. Section 1.
There is one Representative in the House for every thirty thousand citizens and two Senators in the Senate for every State. Representatives serve for two years, Senators six.
Only the House of Representatives may impose tax.
Only Congress is granted any authority to declare war U.S. Constitution Article. I. Section. 8.:
"To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;"
However Congress may not form an army for more than TWO YEARS:
"To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;"
House of Representatives has the sole Power to Impeach and the Senate has the sole Power to try all Impeachments. (Apparently Virginia has more advanced and necessary impeachment law which allows the citizens the right to impeach, all States should consider implementing such a law)
Clearly the founding fathers never expected much from the Federal Government as they only required Congress to assemble once each year. - Section. 4. Clause 2. Full time politicians make business for themselves.
To do business, more than fifty percent of the House or Senate must agree on any matter. - Section 5. Clause 1. The President must approve any law passed by Congress unless more than two thirds of the Congress approve the law. - Section 7., Clause 2.
Problems With Republican Construct
The United States is not a Democracy, it is a Republic and can be subject to a non-representative government takeover: see Democracy or Republic for consideration of the problem.