Congress tried to ban "assault-style" weapons in 1994 but put a 10-year sunset provision in the law. It survived court challenges at the time, but when the 10-year term had passed, the majority control of Congress had also passed — from the Democrats, who had enacted the ban, to the Republicans, who let it lapse.

Since then, all efforts to restrict the sale of such weapons have failed. Even relatively bipartisan attempts at strengthening other restrictions, such as the Manchin-Toomey background check expansion bill in 2013, have fallen short of the necessary supermajority needed for passage in the Senate.

It was not, as President Trump alleged Wednesday, because of a lack of "presidential backup." President Barack Obama supported the bill, as Sen. Pat Toomey, a Pennsylvania Republican, pointed out to Trump. Republicans filibustered the bill, which got 54 votes.

In each case, defenders of gun rights have invoked the Second Amendment, the text that casts a long shadow across all discussions of guns in the U.S. At times, it seems to all but end such discussion.

Parkland changes calculus

But now, the tide is running the other way. The Parkland shootings have created a new moment and a new movement, led by teenagers who survived the tragedy and took their protests to social media and beyond.

Suddenly, even Trump is tossing out ideas about keeping students safe, arming teachers, restraining gun sales through background checks and higher age limits, and even banning accessories such as "bump stocks" that enable nonautomatic weapons to fire rapidly and repeatedly.

And it's still unclear what Trump wants exactly. Republicans on Capitol Hill seem flummoxed by Trump's posture.

After Trump's made-for-cable bipartisan meeting at the White House with members of Congress, Texas Republican John Cornyn, a leader on gun issues in the Senate, seemed to scratch his head.

"I think everybody is trying to absorb what we just heard," Cornyn told reporters. "He's a unique president, and I think if he was focused on a specific piece of legislation rather than a grab bag of ideas, then I think he could have a lot of influence, but right now we don't have that."

He added that he didn't think simply because the president says he supports something that it would pass muster with Republicans. "I wouldn't confuse what he said with what can actually pass," Cornyn said. "I don't expect to see any great divergence in terms of people's views on the Second Amendment, for example."

Ah, and there are those two words again — Second Amendment.

If new restrictions are enacted — a prospect far from certain, as Cornyn rightly points out — they will surely be tested in the courts. There, it will be argued that they infringe on the rights of law-abiding citizens to "keep and bear" firearms.

In other words, they will run afoul of, that's right, the Second Amendment.

Anticipating that, some gun control advocates — and at least one lifelong Republican — want to leap to the ultimate battlement and do it now. They want to repeal, or substantially alter, the formidable amendment itself.

That would seem logical, at least to these advocates. If some 70 percent of Americans want more gun control and the Second Amendment stands in their way, why shouldn't they be able to do something about it?

Someday, it is conceivable, the people and politicians of the United States may be ready for that. But it will need to be a very different United States than we know today.

Why? Because amendments to the Constitution, once ratified, become fully part of the Constitution. Changing or removing them requires a two-stage process that has proved historically difficult.

The Founding Fathers were willing to be edited, it seems, but they did not want it to be easy. So they made the amending process a steep uphill climb, requiring a clear national consensus to succeed.

Why it takes consensus

A proposed amendment to the Constitution must first be passed by Congress with two-thirds majorities in both the House and the Senate.

The two chambers have not achieved such a margin for a newly written amendment to the Constitution in nearly half a century. The last such effort was the 26th Amendment (lowering the voting age nationwide from 21 to 18), and it cleared Capitol Hill in March 1971.

(There has been another amendment added since, in 1992, but it had been written and approved by Congress literally generations ago. More about that curious "zombie" amendment below.)

Even after surviving both chambers of Congress in 1971, the 18-year-old vote amendment still had to survive the second stage of the process — the more difficult stage.

Just like all the other amendments before it, the new voting age had to be ratified by three-fourths of the states. That is currently at least 38 states. Another way to look at it: If as few as 13 states refuse, the amendment stalls.

This arduous process has winnowed out all but a handful of the amendments proposed over the past 230 years. Every Congress produces scores of proposals, sometimes well over 100. The 101st Congress (1989 to 1991) produced 214.

Some deal with obscure concerns; many address facets of the electoral process — especially the Electoral College and the choosing of a president. Many are retreads from earlier sessions of Congress. The one thing most have in common is that they never even come to a vote.

Two that fell short

In 1995, a watershed year with big new GOP majorities in both chambers, two major constitutional amendments were brought to votes in the Capitol. One would have imposed term limits on members of Congress. It failed to get even close to two-thirds in the House, so the Senate did not bother.

The other proposed amendment would have required the federal government to balance its budget, not in theory down the road but in reality and in real time. It quickly got two-thirds in the House but failed to reach that threshold in the Senate by a single vote (one Republican in the chamber voted no).

So even relatively popular ideas with a big head of steam can hit the wall of the amendment process. How much more challenging would it be to tackle individual gun ownership in a country where so many citizens own guns — and care passionately about their right to do so?

Overcoming the NRA and other elements of the gun lobby is only the beginning. The real obstacle would be tremendous support for guns in Southern, Western and rural Midwestern states, which would easily total up to more than enough states to block a gun control amendment.

There have been six amendments that got the needed margins in House and Senate but not the needed margin of support in the state legislatures. The most recent was the Equal Rights Amendment, a remarkably simple statement ("Equality of rights under the law shall not be denied or abridged by the United States or any State on account of sex") that cleared Congress with bipartisan support in 1972 and quickly won nods from most of the states.

But in the mid-1970s, a resistance campaign began and stymied the ERA in many of the remaining states. The resistance then managed to persuade several states to rescind their ratification votes. With momentum now reversed, the ERA died when its window for ratification closed.

Zombie amendments

Other amendments that met similar fates included one granting statehood to the District of Columbia. Like the ERA, the D.C. amendment had a time limit for ratification that expired. But other amendments sent out for ratification in the past did not have a limit, and so might still be ratified — at least theoretically.

The granddaddy of these "zombie" amendments was the very first among the Bill of Rights, which began with 12 items rather than 10. The proposed amendment sought to regulate the number of constituents to be represented by a member of the House, and its numbers were soon outdated. So it has never been ratified and presumably will not be.

The one other amendment originally proposed in 1789 but not ratified as part of the original 10 amendments sat around for generations. Then it caught the attention of state legislatures in the late 1980s, at a time of popular reaction against pay raises for Congress. This amendment stated that a member of Congress who voted for a pay raise could not receive that raise until after the next election for the House of Representatives.

That amendment was dusted off and recirculated, and it reached the ratification threshold in 1992, more than 200 years after it had first been proposed. It is now the 27th Amendment to the Constitution, and the last — at least so far.

A new Constitutional Convention?

If all this seems daunting, as it should, there is one alternative for changing the Constitution. That is the calling of a Constitutional Convention. This, too, is found in Article V of the Constitution and allows for a new convention to bypass Congress and address issues of amendment on its own.

To exist with this authority, the new convention would need to be called for by two-thirds of the state legislatures.

So if 34 states saw fit, they could convene their delegations and start writing amendments. Some believe such a convention would have the power to rewrite the entire 1787 Constitution, if it saw fit. Others say it would and should be limited to specific issues or targets, such as term limits or balancing the budget — or changing the campaign-finance system or restricting the individual rights of gun owners.

There have been calls for an "Article V convention" from prominent figures on the left as well as the right. But there are those on both sides of the partisan divide who regard the entire proposition as suspect, if not frightening.

One way or another, any changes made by such a powerful convention would need to be ratified by three-fourths of the states — just like amendments that might come from Congress.

And three-fourths would presumably be as high a hurdle for convention-spawned amendments as it has been for those from Congress — dating to the 1700s.

Correction: March 1, 2018 12:00 am — In an earlier version of this story, the comma after "A well-regulated militia" was inadvertently omitted from the Second Amendment.Also, an earlier photo caption misidentified ammunition magazines as clips.

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