The Supreme Court is back in session, with several big cases and decisions yet to come on issues of civil rights including the voting rights act, same sex marriage and affirmative action in school admissions. NBC's Pete Williams reports from Washington.

The U.S. Supreme Court will hear arguments Wednesday in a landmark Fourth Amendment case that could clear up almost 50 years of uncertainty over the constitutionality of blood tests that are taken without a suspect's consent.

The case involves a traffic stop in Missouri, but its ramifications could range far wider, potentially rewriting drunk-driving laws in all 50 states.

"It comes down, basically, to are you going to see blood draws every single time someone gets pulled over for a DUI," said Michael A. Correll, a litigator with the international law firm Alston & Bird, who examined the legality of blood draws in the West Virginia Law Review last year.

Because drunk-driving stops are such an everyday occurrence, "it's going to affect a broad area of society," he told NBC News, adding: "This may be the most widespread Fourth Amendment situation that you and I are going to face" for the foreseeable future.

Writing last month in the journal of the Texas District and County Attorneys Association, Lauren Owens, a research attorney for the organization, said, "The outcome of the case could lead to a dramatic increase in the number of DWI cases supported by blood evidence."

The case began in October 2010, when Tyler McNeely of Cape Girardeau, Mo., about 100 miles south of St. Louis, was pulled over for speeding. According to court documents, McNeely was unsteady and failed field sobriety tests, so state Highway Patrol Cpl. Mark Winder asked him to take a breath test.

Even before Supreme Court rules, gay marriage battles rage in the states

When McNeely refused, Winder took him to a hospital, where McNeely refused to take a blood test. Winder told the lab technician to take a sample anyway. The record shows that at no time did Winder seek a warrant compelling the test, which indicated that McNeely's blood-alcohol level was almost double the legal limit.

But McNeely's lawyers persuaded the trial judge to exclude the evidence as a warrantless search in violation of the Fourth Amendment to the U.S. Constitution:

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

Here's where it gets complicated. Earlier in 2010, the Missouri Legislature changed the state's "implied consent" law, which says that if you drive on Missouri's roads, you've automatically consented to take a sobriety test.

The previous language said explicitly that if you refused to take a test, then "none shall be given" and the refusal itself could be used as evidence against you.

The new language left out the four words "none shall be given," re-emphasizing that the driver had consented simply by having gotten behind the wheel in the first place. Winder testified that he had read a journal article about the change and said he made a "conscious decision" not to seek a warrant "due to the law changes."

On appeal, the state argued that no warrant was needed because of a 1966 U.S. Supreme Court ruling in a California DUI case that laid out circumstances under which law enforcement could order a blood test without a warrant.

Supreme Court lets embryonic stem cell research go forward

In general, a person's blood is protected under the Fourth Amendment, Justice William Brennan wrote in Schmerber v. California (.pdf): "Search warrants are ordinarily required for searches of dwellings, and, absent an emergency, no less could be required where intrusions into the human body are concerned."

But Brennan noted that Armando Schmerber, the driver in the California case, had been in an accident. Because the officer had to investigate the scene and make sure Schmerber was taken to a hospital for treatment, "there was no time to seek out a magistrate and secure a warrant" before the driver's body metabolized the alcohol in his system, Brennan wrote.

So Brennan carved out what he called a "stringently limited" exception to the Fourth Amendment's warrant requirement because of the likelihood that evidence — the alcohol in the driver's blood — would be destroyed during the delay. That clause has come to be known as the "exigent circumstances" or "special facts" exception.

Missouri argued that delaying McNeely's blood test while the officer sought a warrant amounted to an exigent circumstance because the alcohol in his blood would be destroyed. McNeely argued that because his case involved a straightforward DUI stop — he wasn't in an accident, unlike Schmerber in 1966 — Winder had plenty of time to seek a warrant.

Missouri's Supreme Court agreed with McNeely in January 2012, writing (.pdf):

The patrolman here, however, was not faced with the "special facts" of Schmerber. Because there was no accident to investigate and there was no need to arrange for the medical treatment of any occupants, there was no delay that would threaten the destruction of evidence before a warrant could be obtained. ... The sole special fact present in this case, that blood-alcohol levels dissipate after drinking ceases, is not a per se exigency pursuant to Schmerber justifying an officer to order a blood test without obtaining a warrant from a neutral judge.

As the court itself noted, Brennan stressed 47 years ago that his analysis was expressly limited to the facts of the Schmerber case, but that hasn't stopped various state and federal courts from referring to it over the years, not all of them reading it the same way.

So in May, the state of Missouri asked the U.S. Supreme Court (.pdf) to step in because "this emerging conflict on a fundamental Fourth Amendment issue will likely continue to divide courts throughout the United States."

Watch US News videos on NBCNews.com

The federal government has sided with Missouri, writing in a friend-of-the-court brief (.pdf) that "the fact that the evidence of intoxication is necessarily leaving the suspect's system provides the required exigency." Prosecutors from across the country joined to file a similar brief (.pdf).

But the American Civil Liberties Union, which is representing McNeely, argued that there were no special circumstances trumping the Fourth Amendment.

In any event, it told the Supreme Court (.pdf), the issue is groundless, because — as he testified himself — the arresting officer ordered the blood test because he thought he could, not because of any "special facts." That means it's "a strange case in which to construe the exigency exception to the Fourth Amendment," the ACLU argued.

The court's decision is likely to come down to one simple question, Correll said: "Did Schmerber create a blanket exception to the Fourth Amendment or didn't it?"

"What does the court indicate the emergency is?" he asked. "Is the emergency the inability to get a warrant in a set period of time, or is the emergency that the blood alcohol is dissipating?"

As for McNeely, he's not off the hook even if he wins. Under a separate law that isn't at issue, his driver's license was revoked because he refused to take the breath and blood tests. And both sides agree that the blood test wasn't the only evidence against him, meaning he could still be convicted of felony drunk driving.

More content from NBCNews.com:

Follow US news from NBCNews.com on Twitter and Facebook