The 26th Amendment
One reason for the intensity of the 1972 presidential election was that it was the first in which all Americans over 18 would be eligible to vote. Before the passage of the 26th Amendment, most states required voters to be 21, while all men over 18 were eligible to be drafted into the army. In the late 1960s, with troop commitments in Vietnam near their peak, the average age of an enlisted man was 19, meaning that the bulk of soldiers in Vietnam could not vote.
There had been previous political movements to lower the voting age: the first such effort was made just after the Civil War. The idea reappeared sporadically during the 20th century, gaining the most support during times of war. During World War II, both Senator Arthur Vandenberg and Representative Jennings Randolph proposed constitutional amendments to lower the voting age nationally. Neither measure was acted upon. Over the next two decades, Jennings Randolph, who later became a senator, proposed a voting-age constitutional amendment ten times. Each measure won a majority of senator's votes, but fell short of the required two-thirds majority. In 1968, both party platforms called for extending voting rights to 18-year-olds, and opinion polls showed a majority of Americans supported the idea. Senator Edward Kennedy proposed legislation to lower the age, and both houses of congress quickly passed the measure. Some states disputed the law's constitutionality, however, and the Supreme Court ruled in Oregon v. Mitchell that congress only had authority to lower the voting age in federal elections.
Sensing the logistical difficulty of maintaining two election systems — one for local and state elections and another for federal elections — both houses of congress moved quickly to pass a constitutional amendment. The 26th Amendment was approved by the senate on March 10, 1971, and was quickly ratified by state legislatures. Three fourths of the state legislatures must approve an amendment before it is ratified, and the 38th and 39th states (Oklahoma and North Carolina) ratified the amendment on July 1. It was the quickest ratification process of any constitutional amendment. Because of the new amendment, eleven million extra voters were eligible in 1972 — citizens who were 18, 19, or 20 years old. Fifty percent of them cast votes that year, a proportion that has never been equaled.