According the Congressional Research Service (CRS) report, Congressional Salaries and Allowances, the allowances are provided to cover "official office expenses, including staff, mail, travel between a Member's district or state and Washington, DC, and other goods and services."
In the House of Representatives
The Members' Representational Allowance (MRA)In the Senate
In the House of Representatives, the Members' Representational Allowance (MRA) is made available to help members defray expenses resulting from three specific components of their "representational duties," those being; the personal expenses component; the office expenses component; and the mailing expenses component.
Members are not allowed to use their MRA allowance to pay any personal or political campaigning expenses. Conversely, members are not allowed to use campaign funds to pay for expenses related to their daily congressional duties.
Members must pay any personal or office expenses in excess of the MRA out of their own pockets.
Each member receives the same amount of MRA funds for personal expenses. Allowances for office expenses vary from member to member based on the distance between the member's home district and Washington, D.C., and average rent for office space in the member's home district. Allowances for mailing vary based on the number of residential mailing addresses in the member's home district as reported by the U.S. Census Bureau.
The House sets the funding levels for the MRA annually as part of the federal budget process. According to the CRS report, this amount decreased from a total of $660 million for fiscal year 2010, to $573.9 million for fiscal year 2012.
In 2012, individual representatives received MRA allowances ranging from $1,270,129 to $1,564,613, with an average of $1,353,205.13.
Most of each member's annual MRA allowance is used to pay their office personnel. In 2012, for example, the office personnel allowance for each member was $944,671.
Each member is allowed to use their MRA to employ up to18 full time, permanent employees and up to four part time or temporary employees. The annual salary of any employee of a member of the House of Representatives is currently limited to $168,411 (in 2013).
Some primary responsibilities of the congressional staffs in both the House and Senate include analysis and preparation of proposed legislation, legal research, government policy analysis, scheduling, constituent correspondence, and speech writing.
All members are required to provide a quarterly report detailing exactly how they spent their MRA allowances. All House MRA expenditures are reported in the quarterly Statement of Disbursements of the House. Since 2009, these reports have been available online at: http://disbursements.house.gov/.
The Senators' Official Personnel and Office Expense Account (SOPOEA)
In the U.S. Senate, the Senators' Official Personnel and Office Expense Account (SOPOEA) is made up three separate allowances: the administrative and clerical assistance allowance; the legislative assistance allowance; and the official office expense allowance.
All Senators receive the same amount for the legislative assistance allowance. The size of the administrative and clerical assistance allowance and the office expense allowance vary based on the population of the state the senators represents, the distance between Washington, D.C. and their home states, and limits authorized by the Senate Committee on Rules and Administration.
The combined total of the three SOPOEA allowances can be used at the discretion of each Senator to pay for any type of official expenses they incur, including travel, office personnel or office supplies. However, expenses for mailing are currently limited to $50,000 per fiscal year.
The size of the SOPOEA allowances are adjusted and authorized within the "Contingent Expenses of the Senate," account in the annual legislative branch appropriations bills enacted as part of the annual federal budget process.
In the fiscal year 2013 legislative branch appropriations bill, the size of the average Senate SOPOEA allowance is $3,209,103, with individual accounts ranging from $2,960,716 to $4,685,632, depending on the population of the senators' states.
According to the CRS, the total size of the Senate SOPOEA allowance decreased from $422.0 million in fiscal year 2001 to $396.2 million in fiscal year 2012.
Senators are prohibited from using any portion of their SOPOEA allowance for any personal or political purposes, including campaigning. Payment of any amount spent in excess of a senator's SOPOEA allowance must be paid by the senator.
Unlike in the House, the size of senators' administrative and clerical assistance staff is not specified. Instead, senators are free to structure their staffs as they choose, as long as they do not spend more than provided to them in the administrative and clerical assistance component of their SOPOEA allowance.
The fiscal year 2013 legislative branch appropriations report allowed $477,874 for each senator's legislative assistance allowance, stating that this amount was equivalent to three staff members being paid $159,291 each.
By law, all SOPOEA expenditures of each senator are published in the semiannual Report of the Secretary of the Senate, available online at: http://www.senate.gov/legislative/common/generic/report_secsen.htm.