(CNN) President-elect Donald Trump is looking for a surefire conservative for the Supreme Court, and Senate Democrats are vowing to fight his choice.

But for all the escalating rancor, this round to replace the late Justice Antonin Scalia could be the prelude to a more consequential battle.

The possibility of a second Supreme Court vacancy in the near future is subtly affecting the strategy of the Republican Trump team in the final stages of selecting a candidate and of Democratic opponents girding for what could be years of political turmoil surrounding the composition of America's highest court.

Scalia, who died last February, was a rigid conservative on social issues such as abortion rights, affirmative action and gay marriage. Trump's replacement would likely be a wash.

But a Trump successor to either of the two eldest justices -- liberal Ruth Bader Ginsburg, who will turn 84 in March, or centrist-conservative Anthony Kennedy, turning 81 in July -- could truly transform the law in America.

Consider the stakes if Justice Kennedy, for example, were to step down in the near future:

He was the decisive, fifth vote to throw out tough abortion regulations in Texas last year. Without him, restrictions on when a woman may end a pregnancy are more apt to be upheld.

Kennedy provided the key vote to affirm a University of Texas racial affirmative action policy in 2016. Without him, admissions practices boosting the chances of African American and Latino applicants to ensure campus diversity could be on shaky legal ground. Such policies have been upheld since 1978.

In one of his most defining moves, Kennedy in 2015 cast the crucial fifth vote and wrote the opinion declaring a right to marriage for gay men and lesbians. Since a 1996 case, Kennedy has taken the lead against discrimination based on sexual orientation.

Some former law clerks and others close to the justice say Kennedy, a Republican appointee now in his 30th year on the bench, has been privately considering retirement. He did not respond to requests to comment on any retirement plans.

For her part, eldest Justice Ginsburg, a 1993 appointee of Democratic President Bill Clinton, has said she would stay on the bench as long as she is healthy. She spoke out against the Trump presidency last summer and is likely loath to give him the opportunity to appoint her successor.

The next oldest justices are Stephen Breyer, a 1994 Clinton appointee who will turn 79 this year, and Clarence Thomas, a 1991 appointee of Republican President George H.W. Bush, who will turn 69.

List of 21 possibilities for Scalia's seat

The current fight began last February when Scalia, 79, was found dead in his bed at a remote Texas hunting resort. Senate Majority Leader Mitch McConnell, a Kentucky Republican, immediately vowed to block any nominee of President Barack Obama, despite nearly a year left in his term.

In March, Obama selected veteran appeals court judge Merrick Garland, and Senate Republicans stuck to the McConnell pledge, refusing even to hold hearings on Garland.

That unprecedented stall has ratcheted up the partisan acrimony awaiting this first Trump nomination. Sen. Jeff Merkley, an Oregon Democrat now chief deputy whip, is among those who have deemed the seat "stolen" by Republicans.

Senate Minority Leader Chuck Schumer, a New York Democrat, has warned of Trump nominees, "If they're out of the mainstream we'll oppose them tooth and nail."

President-elect Trump said on Wednesday that he intends to announce his nomination about two weeks after his January 20 inauguration.

Trump advisers say they have pared his earlier list of 21 possible candidates down to six long-serving federal appeals court judges, including Steven Colloton of Iowa, Neil Gorsuch of Colorado, Thomas Hardiman of Pennsylvania, Raymond Kethledge of Michigan, William Pryor of Alabama and Diane Sykes of Wisconsin. But Trump aides say the selection process remains fluid and others candidates, including those on state supreme courts such as Michigan justice Joan Larsen, are still under consideration.

The possibility of a second opening gives Trump a chance to placate conservative insiders jockeying for their favorite candidates, including GOP senators rooting for home-state jurists. Candidates edged out on this round may be on deck for the next.

Trump is also using the possibility of a second opening to answer advisors, particularly from prior Republican administrations, who have pressed him to broaden his list of candidates to prominent conservative intellectuals. Among those left out were US Appeals Court Judge Brett Kavanaugh and former US Solicitor General Paul Clement, Washington insiders who would likely have made the shortlists of a more traditional Republican president.

Bork and filibusters

Yet, as much as the prospect of a second seat may offer a president more options, the potential for an ideological mismatch and a bruising confirmation fight cannot be dismissed.

The possibility recalls the ordeal that led to the appointment of Anthony Kennedy three decades ago.

When centrist Justice Lewis Powell announced his retirement in June 1987, President Ronald Reagan nominated hardline conservative Judge Robert Bork of the US Court of Appeals for the District of Columbia Circuit.

Then Sen. Ted Kennedy, a Massachusetts Democrat, immediately declared that his appointment would mean America's return to back-alley abortions and segregated lunch counters. The Senate rejected Bork's nomination by a vote of 58-42.

President Reagan eventually turned to then-US Appeals Court Judge Kennedy of California. He was approved on a vote of 97-0 in February 1988.

Democrats controlled the Senate back then. Republicans dominate now (52 seats, to 46 Democrats and 2 independents), which necessarily constrains Democratic chances for defeating Trump nominees. The Senate filibuster -- which requires 60 votes to break -- remains in effect for Supreme Court nominations, but GOP leaders could always decide to invoke the "nuclear option" to break that rule should Democrats not accede to an up-or-down vote on Trump's pick.

Further, Democrats and their liberal allies now researching the backgrounds of Supreme Court candidates know that a succession of controversial appointments naturally tests the opposition's energy and resources.

In 1986, for example, Democrats poured their opposition efforts into Reagan's elevation of then-Associate Justice William Rehnquist to be chief justice and gave Scalia, chosen to succeed him as an associate justice, limited scrutiny. Rehnquist ended up approved on a vote of 65-33; Scalia was approved 98-0.

This time around liberal advocates are hoping that a Democratic show of force on the first nomination serves as a warning to Trump not to put up an uncompromising conservative for a more consequential opening.