SPRINGFIELD, Ill. — The Illinois Supreme Court struck down a law Thursday that reduced the size of civil juries from 12 members to six, finding that the writers of the state constitution had intended to guarantee larger juries when they adopted the current charter in 1970.

Justices voted 5-0 to overturn a 2014 law that determined six jurors should hear civil cases. It was an effort to increase pay for jurors taking time away from jobs and families by spreading the compensation pool among half as many people. Two justices didn’t take part in the decision.

The case in question involved a man and woman who filed a medical negligence suit against five doctors and their employers. The defendants in the case filed a lawsuit arguing the law, which took effect in June 2015, violates the right of trial by jury and separation of powers among branches of government.

Chief Justice Rita Garman noted in writing the court’s opinion that both the U.S. and Illinois constitutions protect the right of trial by jury but that the court gives more weight to the state document. It guarantees “the right of trial by jury as heretofore enjoyed,” and the justices concluded that means to preserve the practice of 12-person juries.

“There is ample evidence that the drafters at the 1970 Constitutional Convention believed they were specifically preserving the right to a 12-person jury when they adopted the current constitution,” Garman wrote. “Transcripts from the constitutional convention debates disclose that the delegates did not believe that the legislature had the authority to reduce the size of a civil jury prior to the 1970 convention.”

The Democratic-controlled Illinois General Assembly OK’d the change in late 2014 — just before Republican Bruce Rauner became governor.

Advocates argue more pay means drawing jurors from a wider cross-section of the community. Trial lawyers say they’re not getting jurors representative of the community.

Opponents claim strong personalities can dominate smaller juries and lead to inconsistent verdicts. And in Illinois Republicans derided the idea as a benefit to Democratic-leaning trial lawyers because smaller juries tend to award more money in damages.

The court’s opinion invalidated the entire law — including the higher pay provision.

The case is Kakos v. Butler.

Online: http://www.illinoiscourts.gov/Opinions/

Contact Political Writer John O’Connor at https://twitter.com/apoconnor . His work can be found at http://bigstory.ap.org/content/john-oconnor .