The all-Republican court's decision comes in response to a first-of-its-kind legal maneuver in which a House committee voted to subpoena Roberson after his scheduled death.

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The Texas Supreme Court late Thursday spared Robert Roberson on the night he was set to die by lethal injection, a rare and head-spinning eleventh-hour decision in one of the most controversial death penalty cases in years.

The all-Republican court's decision comes in response to a first-of-its-kind legal maneuver in which a state House committee voted to subpoena Roberson for a hearing scheduled days after his execution date. It could buy Roberson — who was set to become the first American executed for a conviction involving "shaken baby" syndrome at 6 p.m. Thursday — weeks or months to live as court proceedings continue to play out.

The order caps a whirlwind two-day effort from a bipartisan coalition of lawmakers who feverishly fought to keep a man they believe to be innocent from the execution chamber and riveted the nation's attention on Texas' application of the death penalty.

The House representatives who led the movement expressed relief in a Thursday night joint statement.

"For over 20 years, Robert Roberson has spent 23.5 hours of every single day in solitary confinement in a cell no bigger than the closets of most Texans, longing and striving to be heard," said Reps. Jeff Leach, R-Plano, and Joe Moody, D-El Paso. "And while some courthouses may have failed him, the Texas House has not."

Dueling rulings from the courts

The drama Thursday took off when Leach and Moody successfully asked a Travis County state District Court to temporarily stay the execution to allow Roberson to answer a summons that the House Committee on Criminal Jurisprudence unanimously approved Wednesday.

The Texas Court of Criminal Appeals shortly thereafter overturned that lower court's approval of the lawmakers' request in a 5-4 decision, and minutes later Leach and Moody filed an emergency motion with the Texas Supreme Court to intervene, arguing the Criminal Appeals Court lacked jurisdiction over a ruling made in a civil court.

Leach posted on social media before the state Supreme Court's decision that he was "Praying as if everything depends on God, which it does. But working as if everything depends on us."

The state Supreme Court agreed with the lawmakers, with Justice Evan Young writing in a concurrence that "the underlying criminal-law matter is within the Court of Criminal Appeals’ authority, but the relief sought here is civil in nature, as are the claims that have been presented to the district court."

Roberson's case for a reprieve has drawn widespread support from more than 80 Texas House members as well as from U.S. Supreme Court Justice Sonia Sotomayor, Dr. Phil and others.

After the U.S. Supreme Court rejected a petition to delay the execution around 4 p.m. Thursday, Sotomayor wrote in a statement that "mounting evidence suggests ... Roberson committed no crime at all."

Sotomayor and others have urged Gov. Greg Abbott to grant Roberson a 30-day reprieve, but the governor has remained silent on the case.

More: One day from execution, Texas rejects Robert Roberson's clemency bid in 'shaken baby' case

What the Texas Supreme Court order means

The Texas Supreme Court order came down four hours after the execution had been set to take place, and two hours before the day's clock ran out at midnight. Under Texas law, a death sentence is required to be carried out on the calendar day it has been scheduled.

The action by the state's highest civil court does not weigh into the issues at the heart of Roberson's death sentence, nor does it take Roberson off death row. Instead, Justice Young wrote in the court's ruling, it centers on the constitutional doctrine of separation of powers.

What must be settled are the questions of under what circumstances may one branch of government override decisions of another.

"The underlying criminal-law matter is within the Court of Criminal Appeals’ authority, but the relief sought here is civil in nature, as are the claims that have been presented to the district court," Young wrote. "Whether the legislature may use its authority to compel the attendance of witnesses to block the executive branch’s authority to enforce a sentence of death is a question of Texas civil law, not its criminal law."

The ruling sends the matter back to the state District Court that earlier in the day upheld the legislative committee's subpoena to have Roberson testify.

And Young's order might contain the biggest understatement of a day rich in precedent-setting actions for Texas law.

"These questions implicate the separation of powers at a high level," he wrote. "Some separation-of-powers issues are not justiciable but must be resolved by the other two branches, and this may be such a case.

"We do not have clear precedent on this question; once the question is resolved, future cases would be addressed in light of that resolution."

In the hearing on the motion for an emergency stay, Moody argued that "no agency ... can stand in the way of a legitimate subpoena that's been issued by the Legislature in furtherance of a legitimate legislative purpose."

"People are going to argue about Mr. Robertson's case long after today," Moody said. "But the issue today is, does the Legislature have the authority to call a witness under a legitimate legislative purpose before it and (can) an agency ... simply ignore that that legally issued subpoena?"

The attorney general's office acknowledged the subpoena was "legal and valid" but argued the Court of Criminal Appeals had "exclusive jurisdiction" over the case involving Roberson's death.

Roberson's 'shaken baby syndrome' conviction widely questioned

The condemned man, listed in prison records as Robert Leslie Roberson III, Inmate No. 999442, has been on Texas death row since Feb. 21, 2003. He was sentenced to death in the killing of his 2-year-old daughter, Nikki, 13 months earlier. His lawyers have argued without success in several appeals that the conviction rested solely on what they said was the debunked theory that a baby's life could be snuffed out by the violent shaking of its body.

Brian Wharton, the former police detective who built the case that sent Robertson to death row, said he is persuaded that shaken baby syndrome relies on junk science and has come to believe that no crime was committed in Nikki's death.

Roberson, who has been diagnosed with autism, told authorities at the time of Nikki's death that the child had fallen out of bed and he rushed to the hospital in a desperate effort to save her life.

An agonizing waiting game

The order by Travis County state District Judge Jessica Mangrum to grant the lawmakers' request to allow the Texas House Criminal Jurisprudence Committee to question Roberson next week all but assured that the execution would not be held as scheduled at 6 p.m. Thursday. Instead, it triggered an agonizing waiting game at the prison in Huntsville that houses the death chamber.

Earlier, Roberson was taken from Texas death row in Livingston for the 43-mile trip to Huntsville.

Roberson's sister-in-law Jennifer Roberson told CNN while waiting in Huntsville that the inmate was “angry, heart-broken and furious” that the courts chose not to scuttle his planned execution outright.

“This is an injustice and his blood is on Abbott and Anderson County,” she told the cable network, making reference to the county where Roberson was convicted 21 years ago. “This is not a legal execution, this is murder.”

She said Roberson spent time with his wife, whom he married in recent years, at the prison Thursday morning.

While the legal battle played out in the state capital, Reps. John Bucy, D-Austin, and Lacey Hull, R-Houston, stood outside the prison in Huntsville with protestors, media and Roberson's family members.

Inside the unit, Roberson thanked God and his supporters after he was informed of the stay, a spokesperson of the Texas Department of Criminal Justice told the Statesman. His lawyer nodded to the road ahead and expressed gratitude to the state Legislature.

"The vast team fighting for Robert Roberson— people all across Texas, the country, and the world— are elated tonight that a contingent of brave, bipartisan Texas lawmakers chose to dig deep into the facts of Robert’s case that no court had yet considered and recognized that his life was worth fighting for," Gretchen Sween, an attorney for the Innocence Project, said in a statement. "Thank you to all who have supported Robert, an innocent man on Texas’s death row.”

The hearing where Roberson must appear is set to begin at noon Monday in the Capitol.