Senator or Representative to Congress are established and set out within the U.S. Constitution. There are only three "positive" standing qualifications for the office of U.S. Senator and Representative:
Age—a Representative to Congress must be at least 25 years old; a Senator must be at least 30 years old. Exclusivity of Constitutional Qualifications

Although there may have been some credible minority argument concerning the ability of Congress or the states individually to set additional or different qualifications for federal office from those set out in the Constitution, it is now well-settled that the qualifications established in the U.S. Constitution are the exclusive qualifications for federal office (and are not merely "minimum" qualifications). State laws which have attempted to place requirements on candidacies to Congress that have been deemed to constitute "additional qualifications" to be elected to federal office, such as, for example, requirements of residency in the congressional district from which one is running (for the House of Representatives), disqualification from office of certain convicted felons, additional residency requirements for one to be an "elector" qualified to run for congressional office (or other durational residency requirements), or the disqualification for certain long-term incumbents (term limits), have been found to be unconstitutional when challenged. Judging Qualifications to Office

The U.S. Constitution, at Article I, Section 5, clause 1, expressly delegates to each house of Congress the authority to judge the qualifications for office of its own Members: "Each House shall be the Judge of the Elections, Returns, and Qualifications of its own Members.... " As noted by authorities on parliamentary democracies and the Constitution, since Congress is an independent, separate, and co-equal branch of government, it would not be consistent with its status or functions, nor with the separation of powers principles generally, for a different branch of the government, such as the judiciary, to determine the legislature's make-up:
The propriety of each House being the judge of these matters is very obvious. Article I, Section 2, clause 2, and Article I, Section 3, clause 3 of the Constitution expressly require one to be an "inhabitant" of the state "when elected." Constitutional Disqualifications

In addition to the three standing "positive" qualifications for congressional office in the U.S. Constitution—age, citizenship, and inhabitancy in the state when elected—there are certain "disqualifications" that are based on constitutional provisions. Holding Other Federal Office

The Constitution provides at Article I, Section 6, clause 2, that "no Person holding any Office under the United States, shall be a Member of either House during his continuance in Office." This procedural and administrative authority extends to such things as the form of the ballots, voting procedures, counting votes and certifying winners, the nominating process, and may extend to reasonable requirements for a candidate's name to appear on the ballot, that is, so-called "ballot access" requirements for major party, new party, and independent candidates. Legitimate "ballot access" rules are generally promulgated by states to prevent the proliferation of frivolous candidates, ballot overcrowding and voter confusion, election fraud, and to facilitate generally proper election administration. Various "ballot access" procedures, including filing requirements, filing deadlines, a show of qualifying support by new or minor party or independent candidates, "sore loser" laws and other restrictions on cross-filing, have been found generally to be within the state's purview to "regulate[ ] election procedures " to serve the state interest of "protecting the integrity and regularity of the election process ... ," and when they are found to be within the state's administrative authority over elections, they would be deemed to be not impermissible additional qualifications for federal office even though they create certain procedural hurdles or requirements which a candidate must overcome. The petition-signature requirements for congressional candidates to appear on the ballot in various states have been challenged on both First Amendment and Fourteenth Amendment grounds. 5 U.S.C.
There are three, and only three, standing qualifications for U.S. Senator or Representative in Congress which are expressly set out in the U.S. Constitution: age (25 for the House, 30 for the Senate); citizenship (at least seven years for the House, nine years for the Senate); and inhabitancy in the state at the time elected. U.S. Constitution, Article I, Section 2, cl. 2 (House); and Article I, Section 3, cl. 3 (Senate). The Supreme Court of the United States has affirmed the historical understanding that the Constitution provides the exclusive qualifications to be a Member of Congress, and that neither a state nor Congress itself may add to or change such qualifications to federal office, absent a constitutional amendment. Powell v. McCormack, 395 U.S. 486, 522 (1969); U.S. Term Limits, Inc. v. Thornton, 514 U.S. 779, 800-801 (1995); Cook v. Gralike, 531 U.S. 510 (2001).
The Constitution expressly delegates to each house of Congress the authority to be the final judge of the qualifications of its own Members (Article I, Section 5, cl. 1). In judging the qualifications of their Members, and deciding by majority vote, the House and Senate are limited to judging only the qualifications set out in the Constitution. Powell v. McCormack, supra.
Although the states have no authority to add to the constitutional qualifications for congressional office, the states have the responsibility under the "Times, Places, and Manner" clause of the U.S. Constitution (Article I, Section 4, cl. 1) for administering elections for federal office, including regulating such subjects as ballot design, candidate placement on the ballot, ballot security measures, nomination procedures to appear as a party's nominee on the ballot, and ballot access requirements for independent and new or minor political party candidates. Legitimate "ballot access" rules and regulations, even though they may pose certain administrative requirements on federal candidates, have been upheld when they have been found to be within a state's constitutional authority to regulate the election process, to ensure orderly elections, and to prevent fraud and voter confusion. The states have been allowed to implement rules which, for example, prevent over-crowding and confusion on the ballot by requiring a minimum show of public support to appear on the ballot, by prohibiting such things as dual candidacies on the ballot, and by implementing "sore loser" laws that bar a candidate on the general election ballot from appearing as an independent if that candidate had lost a party primary. Such administrative requirements have not been deemed to be additional "qualifications" to run for office. However, requirements that are more than merely administrative and procedural or measures to protect ballot integrity have been found to be unconstitutional as additional qualifications for office. Examples include requirements for congressional candidates to live in the congressional district (and not just the state), durational residency requirements, ineligibility of convicted felons, and disqualification of incumbents (term limits).
This report updates an earlier CRS report, and will be revised as decisions, rulings, and/or events warrant.