The question of what rights criminal defendants have to confront those testifying against them has divided the Supreme Court so deeply and for so long that the justices hardly need the lawyers arguing before them.

That was apparent Tuesday, when the court again considered whether those who conduct forensic tests for law enforcement must be available for questioning at trial. It is ground so familiar that at one point, Justices Antonin Scalia and Stephen G. Breyer simply debated the point themselves, rather than asking questions of a lawyer representing the defendant.

Since 2004, a narrowly divided court has decided a string of cases interpreting the Sixth Amendment’s confrontation clause — which gives the accused the right “to be confronted with the witnesses against him”— in favor of criminal defendants. The rulings defy the court’s usual divide, with the conservative Scalia and the liberal Ruth Bader Ginsburg taking the lead in advocating the defendants’ argument.

Prosecutors have said that requiring a live witness to testify about how forensic tests are performed is costly and cumbersome. Forty-two states supported the prosecutors in this case. In past cases, Breyer has sided with conservatives such as Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. in agreeing with the government.

That lineup remained the same Tuesday as the court considered the case of convicted rapist Sandy Williams of Chicago. After a woman was raped in 2000, police recovered DNA evidence and sent it via FedEx to Cellmark Diagnostic Laboratory in Maryland. The company returned the samples later, along with a male DNA profile it developed from the evidence.

Police had arrested Williams on a separate charge, had withdrawn blood and had created a DNA profile. A computer matched the two profiles, the victim picked Williams out of a lineup, and he was eventually convicted and sentenced to life in prison.

At the trial, state forensic DNA analyst Sandra Lambatos testified about how law enforcement made the match. But because the Cellmark report wasn’t introduced as evidence, the judge said the state did not have to produce an analyst from the lab to testify about how it performed the tests.

Scalia and Ginsburg said Lambatos’s testimony was an attempt to introduce the lab results that indicted Williams without admitting the report into evidence.

“I don’t understand how Lambatos’s testimony can be independent of the test results supplied,” Ginsburg told Cook County State’s Attorney Anita Alvarez. “It is entirely dependent on them.”

Added Scalia: “We don’t know how good the individuals who did the test were. And that’s why it’s up to the state to bring forward testimony saying what the lab did. And the only testimony they brought forward was the testimony of this witness who was not there.”

Breyer worried that if it became too difficult for prosecutors to present forensic evidence, it would “push the system in the direction of relying on less reliable eyewitness testimony rather than more reliable technical laboratory DNA-type evidence.”

Roberts suggested that defense lawyers could bring out in cross-examination that the person testifying to the DNA match could not vouch for how the tests were performed, raising doubts about their reliability.

Justice Anthony M. Kennedy, who has been a vigorous dissenter in the rulings in favor of defendants, seemed to take a different approach in this case. He indicated that Lambatos was doing more than simply testifying about how the match was made. “The key actor in the play, the Hamlet in the play, is the person who did the test at Cellmark,” Kennedy said. “And she or he is not here.”

The case is Williams v. Illinois.