WASHINGTON (Reuters) - U.S. senators on Wednesday are set to probe Brett Kavanaugh, President Donald Trump’s nominee to fill a vacancy on the Supreme Court, over his views on abortion as his confirmation hearing moves into its second day. Here is why it matters:

U.S. Supreme Court nominee judge Brett Kavanaugh is sworn in during a Senate Judiciary Committee confirmation hearing on Capitol Hill in Washington, U.S., September 4, 2018. REUTERS/Joshua Roberts

What is Roe v. Wade?

Roe v. Wade is a landmark U.S. Supreme Court ruling from 1973 that found women have a constitutional right to obtain abortions. The ruling meant that states could not ban abortions, leading to an immediate and sustained backlash from social conservatives that continues to this day.

Why is abortion a key issue in the confirmation hearing?

Kavanaugh could potentially be the deciding vote on overturning the 1973 decision. The court is currently split 4-4 on ideological lines. Justice Anthony Kennedy, a conservative whom Kavanaugh would replace, disappointed conservatives by backing abortion rights in two key cases. Conservatives hope Kavanaugh will seal a majority that would at least be willing to curb abortion rights if not overturn Roe.

What is stare decisis?

Senators are likely to question Kavanaugh in detail on his willingness to question long-standing Supreme Court precedents.

The justices are generally reluctant to overturn precedent, citing a legal doctrine - known as stare decisis - that counsels against it in order to help the court stay evenhanded and appear less politicized.

But the court does overturn its precedents. In June, the conservative majority did so in a major labor case, ditching a 1977 ruling that non-union members could be required to make payments to unions to pay for collective bargaining representation.

What has Kavanaugh said about stare decisis?

At a 2016 event at the American Enterprise Institute, Kavanaugh said: “It’s appropriate that the court overrule precedent in some circumstances.” But he expressed frustration about the lack of a neutral formula for determining when a case should be nixed.

“The hard question is when is it appropriate and when is it not. I would love to have a perfect answer to that question,” he said. Kavanaugh was not specifically asked about Roe v. Wade.

What is Kavanaugh’s record on abortion?

During his decade as a judge, Kavanaugh has not ruled directly on abortion but has signaled sympathy for legal arguments made by anti-abortion advocates.

Last October, he was on a panel of judges that issued an order preventing a 17-year-old illegal immigrant detained in Texas by U.S. authorities from immediately obtaining an abortion. That decision was overturned by the full appeals court and she had the abortion.

In a 2017 speech, Kavanaugh praised several opinions written by former Chief Justice William Rehnquist, including his dissent in Roe v. Wade.