WASHINGTON — The Supreme Court ruled Monday that states could not set term limits for members of Congress, saying American democracy was built on the principle that individual voters choose who governs and for how long.

In a 5-4 decision, the court found that the states do not have the constitutional authority to regulate the tenure of federal legislators. The ruling effectively overturns term limit laws in 23 states, including Maine, and makes amending the Constitution the only sure means of restricting incumbency.

“(A)llowing the several states to adopt term limits for congressional service would effect a fundamental change” in the Constitution, Justice John Paul Stevens wrote for the court. “Any such change must come not by legislation adopted either by Congress or by an individual state, but rather … through the amendment procedures.”

He said a “patchwork” of state tenure qualifications would undermine the uniformity and national character of the U.S. Congress that the Founders sought. The Constitution lists only three qualifications for members of Congress, relating to age, residency and citizenship.

Although the merits of term limits have been debated since the nation’s beginning, this was the first time the issue had come before the Court. The decision seems likely to refocus and reinvigorate one of the most contentious political debates of recent years.

House Republicans made term limits a prominent feature of their “Contract with America” in the 1994 elections. In March, however, the House defeated four different versions of a constitutional amendment for term limits as veteran Republican lawmakers joined Democrats in opposition.

A constitutional amendment requires a two-third vote in each chamber of Congress and approval by three-fourths, or 38, of the states.

None of the state laws mandating congressional term limits — all of which were passed since 1990 — has ever been enforced, and Monday’s ruling essentially invalidates them all. The decision will not affect state laws limiting the tenure of state legislators.

The case decided Monday involved an amendment to the Arkansas state constitution that would have prohibited a candidate from appearing on an election ballot after serving three terms in the House of Representatives and two terms in the U.S. Senate. Such veteran officeholders could only have been elected as write-in candidates.

Expressing the often angry tone of the term limits advocates, the preamble to the Arkansas rule asserts that “entrenched incumbency … has led to an electoral system that is less free, less competitive, and less representative than the system established by the Founding Fathers.”

Arkansas officials and U.S. Term Limits Inc., a lobbying group that intervened on the state’s side, argued that a constitutional provision that gives states authority to regulate the time, place and manner of congressional elections provides broad leeway for ballot restrictions on incumbents. They also asserted that states have broad authority to regulate elections under the 10th Amendment, which says that powers not delegated to the federal government in the Constitution “are reserved” for the states.

The case brought by Bobbie E. Hill, a member of the League of Women Voters, countered that the age, citizenship and residency qualifications specifically delineated in the Constitution for members of Congress are exclusive.

Trying to put the best face on Monday’s defeat, Arkansas Attorney General J. Winston Bryant compared the nationwide term limits movement and its grass-roots support to women’s suffrage. “That took a number of years to come to fruition and I believe the same will happen with term limits.” John G. Kester, representing U.S. Term Limits Inc., which intervened on the state’s behalf, added, “We came within one vote of winning the whole war.”

Monday’s ruling surprised lawyers on both sides of the case with its close vote and the strikingly different views of state power that it reflected.