Congress has four functions: lawmaking; representation; constituent services; and oversight.
Congress is given the power to establish broad national policies, a power known as lawmaking.
Deliberation is a crucial part of this process. The founders intended Congress to be where issues were deliberated and dissected. Deliberation was meant to create consensus and build majority support for policies.
However, deliberation has shifted to the media and executive branch.
Members of Congress represent Americans.
There are two views of how Members of Congress ought balance their obligations to their constituents and to their nation:
Trustee view states that your role is to think of the country as a whole and not just your congressional district. For example, if a Virginian believes a policy is best for the country but not for Virginia, then that person ought support the policy. The trustee view occurs mostly in the Senate, since Senators are re-elected every six years and a disfavored policy from four or five years ago will not be well remembered. The trustee view states that the rep represents whole of society
Delegate view is that the Member of Congress represents preferences of constituents. This best reflects the approach in the House of Representatives because representatives are up for re-election every two years. Their constituents will likely remember very clearly that representative's voting record -- and whether or not it was supported.
Constituent Services (Casework)
Casework is assisting constituents with their federal government dealings.
This has a tremendous impact on people's votes. It reflects favorably on that constituent's vote if -- upon request -- their Member of Congress has a streetlight installed, pothole filled or relative granted a VISA.
Another example of constituent services is flag requests. A Member of Congress will have a flag flown over the Capitol and then shipped to you for a small fee.
Oversight allows Congress to make sure its intentions are carried out by other branches of government.
Oversight is managed through committees and subcommittees. Congress reviews actions of the executive branch and makes sure that its laws are carried out properly. For example, a new speed limit law passed by Congress must be executed by the executive branch. This requires changing every single impacted speed limit sign.
Congress usually uses vague language. For example, Congress created the Occupational Safety & Health Administration (OSHA) "to assure safe and healthful working conditions for working men and women." OSHA then must create laws ranging from exposure to paint thinner to hard-hats. Congress then must provide oversight over OSHA to make sure laws enacted reflect Congress' intentions.
Oversight creates a constitutional struggle between Congress and the executive branch.
However, sometimes it is forsaken when the president is of the same party as the Congressional majority. This is an example of politicians placing their party over the Constitution.