Stuart Goldstein still has the red-and-white bumper stickers and other artifacts from 1969, when he helped persuade New Jersey lawmakers that 18-year-olds should be able to vote.

He was 18 himself then, working with two other college students, David DuPell and Ken Norbe, to build a political network that grew to 10,000 volunteers. They took students to Trenton in busloads and even sneaked into a Richard Nixon rally seeking his support. Theirs was an early salvo in a movement that would end in 1971 with the ratification of the 26th Amendment, which lowered the voting age to 18 from 21.

Fifty years later, there is a nascent movement to change the voting age again — this time to 16 — but there are some big differences between the efforts.

Then, liberal and conservative activists united behind a powerful argument that went back to World War II, when President Franklin D. Roosevelt lowered the draft age to 18: Young people were being conscripted to fight America’s wars but couldn’t vote in its elections.

Today, there is no similarly popular argument. Indeed, a recent poll found that 75 percent of registered voters opposed letting 17-year-olds vote, and 84 percent opposed it for 16-year-olds. In March, when Representative Ayanna Pressley of Massachusetts proposed a 16-year-old voting age amendment to House Democrats’ sweeping voting rights bill, it failed 126 to 305, with almost half of her fellow Democrats voting against it and only one Republican in support.

Opponents in both parties have expressed doubts that 16-year-olds are mature enough to vote. But local, youth-led campaigns to lower the voting age have persisted since at least 2013, when Takoma Park, Md., gave 16- and 17-year-olds the right to vote in municipal elections.

The New York Times recently spoke with activists from the movement 50 years ago, and people on different sides of the issue today, about the cause and the challenges of lowering the voting age.

1969: ‘Old enough to fight’

By the time New Jersey took it up in 1969, the voting age had been on the national radar for decades because of the draft. Through World War II, Korea and the early years of Vietnam, every president suggested it should change. But it didn’t — until the 1960s knocked American politics off its axis.

The activism of the era made it easy to mobilize liberals and students, many of whom were already involved in the antiwar and civil rights movements. “People were pretty revved up during that time to get involved in something,” said Mr. DuPell, who started the New Jersey campaign and recruited Mr. Goldstein and Mr. Norbe to join him. But campus unrest and violent protests helped fuel pushback that they were too immature to vote.

“It was kind of an uphill battle for us trying to convince people young people were responsible, because it was an era when, from a national political point of view, the national leaders were pitting young against old,” Mr. Goldstein, now 68, said. “Our thing was, ‘We’re going to try and work within the system.’ There was all this tumult going on across the country. We didn’t think that would help us convince people that they should lower the voting age.”

In April 1969, the Republican-led New Jersey Legislature approved a state constitutional amendment to lower the voting age to 18. And when summer came, Mr. DuPell, Mr. Goldstein and Mr. Norbe went home to build their organization — they called it the Voting Age Coalition Inc. of New Jersey — and round up support for the voter referendum needed to ratify the amendment.

They appointed county leaders, who appointed municipal leaders. They sold membership cards for a dollar and told the buyers to recruit 10 volunteers apiece. When President Nixon came to campaign for William Cahill, who was eventually elected governor, Mr. Goldstein and Mr. DuPell forged press credentials and sneaked into the rally with a sign seeking Nixon’s endorsement. Mr. Goldstein recalled that Secret Service agents carried him out, but their sign ended up in a front-page photo the next day.

Similar efforts were bubbling up in other states. Sometime in the spring, a group of students in Ohio contacted the New Jerseyans and asked if they, too, could use the “Voting Age Coalition” name. By January 1970, students in 13 states were organizing to lower the voting age.

Voters in New Jersey rejected their amendment, and the Voting Age Coalition started trying to lower the age to 19 instead. But it soon became clear that the momentum in Washington, driven by the combined force of the states, was building faster.

It was then, after Congress passed the 26th Amendment in March 1971, that the grass-roots structure built in 1969 paid off. The Voting Age Coalition got New Jersey legislators to ratify the amendment, which swept across the country faster than any previous constitutional amendment, gaining the necessary support from three-quarters of the states in just 100 days and becoming law on July 1, 1971. Governor Cahill gave Mr. DuPell the pen he had used to sign it.

2019: ‘Skin in the game’

While the upheaval of the 1960s was centered on college campuses, today’s activism is often found in high schools, and that shift has informed voting-age campaigns.

Just as college students did 50 years ago, many high schoolers are pointing to broad social turbulence that affects them — gun violence, for instance — and are seeking to influence the political process by voting. They note that they are already involved through movements like March for Our Lives, which teenagers created after the Marjory Stoneman Douglas High School shooting in Parkland, Fla., in 2018.

“So far as I’m concerned, there’s nothing charitable about this amendment,” Ms. Pressley said. “They have earned their right at the table.”

This argument has been gaining some traction in liberal pockets of the country, but nationwide, it is not popular. Among Republicans, a common reaction to Ms. Pressley’s amendment was that it was a ploy to add Democrats to the electorate: Generation Z, which includes today’s 16- and 17-year-olds, leans much more liberal than older generations, especially on social issues.

A broader objection concerns maturity.

“We don’t allow a 16-year-old to buy a beer, and the decision making is because of their ability to reason at that age,” Representative Mark E. Green, Republican of Tennessee, said on the House floor in March. “And now the other side wants to grant a 16-year-old the ability to decide the future of the country. I think this is foolish.”

Jennifer C. Braceras, a senior fellow at the conservative Independent Women’s Forum, argued in a Boston Globe op-ed that 16-year-olds “don’t have enough skin in the game.”

“At 16, most kids have little awareness of politics, civics or American history, and they have little life experience to inform their decisions,” Ms. Braceras wrote, adding, “Most don’t even pay for their own cellphones — let alone groceries, rent, utility bills or property taxes.”

About a dozen countries — including Argentina, Austria, Brazil and Ecuador — allow voting at 16, but the vast majority allow it at 18.

Mr. Goldstein supports the current effort, though he said part of him worried that lowering the voting age to 16 could lead to 16-year-olds serving in the military. But Mr. DuPell was skeptical.

At 18, “people are out of high school, they’re working, they’re drafted, they’re full participants in our society, and if you’re a full participant, then you should be allowed to vote,” he said, summarizing the argument 50 years ago. “I want to know what the rationale is.”

As expressed by the teenagers leading local voting-age campaigns, the rationale is that between climate change, gun violence, student debt and other issues, they do have enough skin in the game.

“The 16-year-olds right now will be the ones who live with the consequences of the choices the adults make right now,” said Vikiana Petit-Homme, 17, a high school senior in Boston who has been lobbying Massachusetts to let municipalities lower voting ages for local elections.

They also note that 16 is when Americans can work without a limit on their hours and, in most states, drive.

“A lot of 16-year-olds are working and getting taxed,” said Ema Smith, 19, a freshman at Yale who, in high school, helped lead a successful campaign to lower the voting age for local elections in Greenbelt, Md. “People tend to focus on at 18 you can join the military, but there are a lot of things happening at 16.”

Advocates acknowledge that they lack broad public support, and that they are at a disadvantage compared with their counterparts 50 years ago because they don’t have the “old enough to fight, old enough to vote” argument.

Still, Brandon Klugman, manager of the Vote16 campaign at Generation Citizen, which has aided local efforts in places like San Francisco and Washington, said that in 1939, support for lowering the voting age was near where it is now: 17 percent. By 1967, that number was 64 percent.

“This is really early days for this conversation,” he said.