This image provided by the Texas Department of Motor Vehicles shows the design of a proposed Sons of Confederate Veterans license plate. The Supreme Court will weigh a challenge to Texas’s decision to refuse to issue the license plate. (AP)

Supreme Court justices often think that their colleagues have stumbled to the right outcome in a case, even though they took the wrong path to get there. When that happens, a justice will write a concurring opinion, pointing out what he or she thinks is the correct legal reasoning.

That’s what Justice David H. Souter did in 2009, his last year on the court. It was a tricky First Amendment case involving a little-known religion called Summum, which wanted to place a monument displaying its Seven Aphorisms next to the Ten Commandments and other monuments in a public park in Pleasant Grove, Utah.

In a splintered and somewhat complicated opinion, the court ruled against Summum. It pointed out reasons that the monuments in the park should be seen as speech by the government, not by private individuals, and thus said the city was free to accept some and reject others.

Souter agreed with the outcome in the case but was nervous about going too far. He proposed a simpler rule: asking “whether a reasonable and fully informed observer would understand the expression to be government speech.”

Souter’s concurrence was what is sometimes called a “lone ranger” — no other justice signed on to it. But it is at the heart of another First Amendment challenge the Supreme Court will consider Monday, about the free-speech rights of those who want special messages on their government-issued license plates.

The case comes from Texas, where the Department of Motor Vehicles Board turned down a request for a specialized plate from the Sons of Confederate Veterans, whose insignia contains the Confederate battle flag. The Texas board members have authorized hundreds of special requests — a Texan can celebrate “Dr. Pepper” on his license plate — but rejected this one after receiving tens of thousands of comments about the design, most of them negative.

But a panel of the U.S. Court of Appeals for the 5th Circuit employed Souter’s test to decide the board was wrong. A reasonable observer would conclude that the driver of the vehicle, not Texas, was the speaker honoring the Civil War dead, the panel decided.

And it said the board members had engaged in viewpoint discrimination by adopting the negative view of the Confederate flag as a symbol of racism and slavery, rather than the organization’s view that it was simply honoring ancestors who had fought bravely in a lost cause.

Specialized license plates have bedeviled lower courts for years, as states seeking the additional revenue the plates bring have struggled over whether to allow messages with which not everyone agrees.

Most of the legal battles have been over abortion, as “pro-choice” and “choose life” forces seek a spot on the tags. But the Richmond-based Sons of Confederate Veterans has been litigious as well.

Virginia and Maryland are under court order to issue the plates. Befitting its border-state status, Maryland has sold only 409 vehicle plates and 61 motorcycle plates since it started offering them in 1995 — it was a plan to recall the plates that got the state in trouble with federal courts.

Virginia did not want to allow the organization’s logo on the plates but lost that battle in the U.S. Court of Appeals for the 4th Circuit in 2002. As of last month, 1,343 vehicle tags and 259 motorcycle plates had been registered.

So a “reasonable and fully informed observer” stuck in traffic behind such a plate on Rockville Pike or Glebe Road would have the opportunity to decide whether what she is seeing is a government endorsement or the driver’s message.

In its brief to the court, Texas insists that “the speech that appears on state-issued license plates is government speech” and all agree that the Constitution “allows a state to select the messages, symbols, and viewpoints that it is willing to publicly support.”

It adds: “Texas is not willing to propagate the confederate battle flag by etching that image onto state-issued license plates that bear the state’s name.” The lesson of the Summum decision was about the government’s rights, Texas said, not Souter’s lone concurrence.

Most observers say that the Supreme Court must find that license plates are government speech for the state to have a chance. But Texas argues that even if the messages are seen as private speech, the state has not engaged in viewpoint discrimination.

Texas hasn’t taken sides on the matter of the Confederate flag, and besides, governments are not required to adopt messages with which they disagree.

“The District of Columbia’s license plates include the phrase ‘Taxation Without Representation’; the Speech Clause does not require the district to begin offering license plates with messages that praise the district’s lack of voting representation in Congress,” Texas argues.

But the veterans say that it could not be more clear that it is the driver that has picked one of the more than 350 messages that Texas offers.

“No speech would occur but for a private individual, non-profit, or business designing the message and the driver attaching his or her chosen plate to the bumper and exiting his or her driveway,” the organization argues.

It says the board simply buckled under pressure and that the Texas code that allows it to “refuse to create a new specialty license plate if the design might be offensive to any member of the public” is far too broad.

Besides, the organization says, Texas is probably the last place in the country that should be squeamish about displaying the rebel flag.

Jan. 19 is an official state holiday: Confederate Heroes Day. There are numerous monuments to Confederate figures at the state Capitol, the challengers contend, and the battle flag that Texas so fears etching on license plates is on sale at the capitol gift shop.

“The state apparently does not believe that the ‘message’ of the Confederate flag is offensive to the public, or, if it is offensive, the state certainly does not shy away from its expression because of such offense,” the group argues.

Paulina Firozi contributed to this column.