Sometime between now and the Fourth of July, the Senate plans to revisit what over the course of 17 years has become a seasonal rite of patriotism on Capitol Hill: a vote on whether to amend the Constitution to ban protesters from burning the American flag.

Each time, the arguments on both sides are passionate. Each time, the support needed to move ahead with an amendment falls short.

But this year could be different, as two important trends cross paths.

For one, proponents of the amendment appear to have more support than ever in the Senate. They say they are within one vote of the two-thirds majority they need. The House already has backed the amendment. A majority of Americans say they support a flag amendment, and over time all 50 states have passed some form of resolution urging Congress to act.

“We believe once the amendment moves off of Capitol Hill it will be the swiftest-ratified amendment in the history of the nation,” said Marty Justis of Indianapolis, a Navy veteran and executive director of the Citizens’ Flag Alliance, which for years has led the campaign. He and other supporters will be back on Capitol Hill on Wednesday _ Flag Day _ trying to round up and lock in support.

At the same time, some polling indicates Americans’ once-robust support for a flag amendment is waning and could be tough to recapture for many years if it slips below 50 percent. That has amendment opponents _ a mix of liberals, free-speech activists and conservatives who believe the Constitution should never or almost never be amended _ determined to stave off Senate passage.

“This is very generational _ basically, if this doesn’t pass the next Congress or two, it’s a dying issue,” said Terri Ann Schroeder, a senior lobbyist for the American Civil Liberties Union, which opposes the amendment as an infringement on free speech.

The dynamics of what’s going on in the Senate this year may have as much to do with the histories of the individual lawmakers as with the pressure to be patriotic on Capitol Hill post-9/11 and in the midst of the Iraq war.

Some longtime opponents of the amendment retired in recent years, replaced by newcomers whose campaign platforms included support of a flag amendment, or by former House members who supported the amendment there.

The lower chamber has voted on the amendment more frequently than the Senate, and with less consequence; its fate always has been up to the Senate. The vote Republican leader Bill Frist of Tennessee has promised this year would be the Senate’s first vote on the proposed amendment since 2000, when it fell four votes shy.

If senators’ past votes and public statements are a fair indicator, there are now 66 proponents. With all 100 members present, that would leave them one vote shy of the 67 votes needed to move ahead. But if one or two opponents are out at any given time, reducing the two-thirds threshold to 66 votes or fewer, backers of the amendment could make a go at it.

The divisions on the issue are not entirely partisan. Activists count 14 Democrats among the 66 proponents. Of 34 senators believed to oppose the amendment, three are Republicans, including GOP Whip Mitch McConnell of Kentucky.

Meanwhile, the Gallup organization pegged national support for a flag amendment at 71 percent in 1989, but found support had slipped to 63 percent by 1999, and to 55 percent by last summer.

Gallup’s numbers are not uniformly accepted. The First Amendment Center, a free-speech advocacy organization, pegs support at below 50 percent. The Citizens’ Flag Alliance has just conducted its own polling in 10 states, meanwhile, and claims Gallup’s and the First Amendment Center’s polls both are wrong and that support for protecting the flag is really as strong as it’s ever been.

In the 1960s and ’70s, opposition to the Vietnam War triggered its share of flag burnings back home.

But the push for an amendment that gives Congress the express power to pass laws banning flag desecration began far later, in 1989, as the Cold War was coming to an end. The trigger was a 5-4 ruling by the Supreme Court invalidating a Texas law used to convict a man who in 1984 burned a flag in protest of Republican policies.

The high court said flag burning is protected as an expression of free speech. Congress responded with the Flag Protection Act, a federal law banning flag burning. In 1990, the Supreme Court overturned that law as well.