U.S. President Donald Trump will reveal his nominee to the Supreme Court tonight, a major political appointment that's expected to tilt the nation's highest bench toward the right and affect legislation for at least a generation.

Trump's selection for the lifetime seat will likely be plucked from a short list of three conservative favourites, including a federal appeals court judge from Alabama who slammed the landmark 1973 abortion decision Roe vs. Wade as an "abomination," a Colorado appeals court judge on the 10th circuit billed as a "natural successor" to the late justice Antonin Scalia, and a Pennsylvania appeals court judge who served with Trump's sister.

Why does the Supreme Court matter?

The incoming justice will be entrusted with extraordinary powers as one of nine ultimate guardians of the U.S. Constitution, and is anticipated to be a conservative anchor on a bench currently split between four conservatives and four liberal-leaning justices.

The Supreme Court is a powerful body because so many laws are challenged — whether they're rulings on same-sex marriage, abortion, health care or immigration. Disputes between the executive branch and Congress often end up in the Supreme Court for a final say on constitutional law.

Who's in the running?

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From left, three possibilities for U.S. President Donald Trump's Supreme Court nominee: William Pryor of the U.S. Court of Appeals, 11th Circuit; Neil Gorsuch of the Court of Appeals, 10th Circuit; Thomas Hardiman, of the U.S. Court of Appeals for the Third Circuit. (Associated Press)

Trump will have probably selected someone from a list of 21 conservative-approved names suggested by organizations like the Heritage Foundation and the Federalist Society.

Three names have emerged as front-runners: William Pryor of the U.S. Court of Appeals for the 11th Circuit, Neil Gorsuch of the 10th Circuit and Thomas Hardiman of the Third Circuit.

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The late Justice Antonin Scalia, in a 2010 file photo, died last February, leaving a vacant seat on the Supreme Court. (Kevin Lamarque/Reuters)

Any of the three should please Republicans as a worthy replacement for Antonin Scalia, who followed an "originalist" interpretation of the constitution's original meaning and intent. Scalia died of a heart attack last February.

Whoever fills his vacancy will be "a great addition and a fine successor," says Elizabeth Slattery, a legal fellow with the conservative-leaning think-tank the Heritage Foundation.

"Any of them would carry on Justice Scalia's legacy of originalism and textualism."

Liberal groups are less certain.

"The president, throughout the campaign and now as president, has repeatedly made clear he intends to name an individual who is extreme," says Dan Goldberg, legal director of the progressive advocacy group Alliance for Justice.

"This individual, who does not share the constitutional values of the American people, and who would gut critical legal protections the American people rely on."

Who is William Pryor?

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Judge William Pryor of the U.S. Court of Appeals, 11th Circuit, speaks in Washington on Nov. 17. (Cliff OwenéAssociated Press)

The 55-year-old former Alabama attorney general is an outspoken social conservative who has said he opposes legalized abortion.

He notably called Roe vs. Wade the "worst abomination in the history of constitutional law," an opinion he doubled down on when he was up for a federal judgeship, telling a Senate confirmation panel: "Not only is the case unsupported by the text and structure of the constitution, but it has led to … the slaughter of millions of innocent unborn children."

That declaration demonstrated Pryor "is a man who has courage of conviction," Slattery says.

Goldberg characterizes Pryor as a right-wing "extremist."

He cites Pryor's positions during his time as an Alabama attorney, when he advocated for criminalizing "homosexual sodomy," as well as Pryor's defence of Alabama's practice of handcuffing prisoners to hitching posts under the hot sun for refusing to work on chain gangs.

Who is Neil Gorsuch?

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Judge Neil Gorsuch sits on the 10th U.S. Circuit Court of Appeals. (U.S. Circuit Court of Appeals via Associated Press)

The 49-year-old Colorado native joined the U.S. Court of Appeals for the 10th Circuit after being nominated to the judgeship by President George W. Bush in 2006.

His strong opinions supporting religious liberty appeal to many conservatives. He has sided with the Christian-founded employer Hobby Lobby over its refusal on faith grounds to provide coverage for contraceptives to workers, as mandated by the Affordable Care Act.
 
Slattery says Gorsuch is widely seen as "a natural successor" replicate Scalia's embrace of originalism — the doctrine that attempts to find meaning in the constitution by interpreting it strictly as the Founding Fathers would have intended 230 years ago.

"Trump said he wanted to nominate someone who would be in the mould of Scalia," Slattery says. "Gorsuch is about as close as you can get."

Goldberg worries Gorsuch's record has shown too much support for corporations ahead of women's and workers' rights.

"His record demonstrates that he would be Wall Street's best friend," he says. "He would put the agenda of huge corporate interests and special interests ahead of the American people."

Who is Thomas Hardiman?

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Judge Thomas Hardiman, of the U.S. Court of Appeals for the Third Circuit, is seen in Washington on Nov. 17, 2016. (Cliff Owen/Associated Press)

Judicial raves for Hardiman are coming from someone close to Trump: The president's sister.

Maryanne Trump Barry, a respected senior judge on the Third Circuit in Pennsylvania who served alongside Hardiman, has reportedly spoken in favour of appointing him to the high court. The two judges would have had a close working relationship sitting and deciding cases together since Hardiman's appointment by Bush in 2007.

Republicans are viewing the advice from Trump's older sibling with caution.

"She's considered quite liberal," Slattery says. Former Republican presidential candidate Ted Cruz described her as a "hard-core pro-abortion liberal judge."

Hardiman, who speaks fluent Spanish and married into a prominent Democratic family, is not seen as intensely conservative, though his notable rulings have favoured protecting gun rights and the rights of law enforcement.

The 51-year-old Massachusetts native is regarded as an up-and-coming jurist who, unlike his would-be contemporaries on the Supreme Court, does not have an Ivy League pedigree. Hardiman graduated from the University of Notre Dame and Georgetown University's law school.

Can Democrats stop the nominee?

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Members of the Supreme Court hold their hands over their hearts as Jackie Evancho sings the national anthem at the inauguration ceremonies for Donald Trump on Jan. 20. (Rick Wilking/Reuters)

Democrats will try to filibuster, no matter who Trump's pick is — payback for how the Republicans blocked former president Barack Obama's nominee Merrick Garland last year.

Democratic senators in the 100-member upper chamber have 46 seats to the Republicans' majority of 52. (The remaining two senators are left-leaning independents.) The Republicans need 60 votes for a supermajority in order to confirm the Supreme Court pick.

Senate majority leader Mitch McConnell has pledged, "The nominee will be confirmed," a declaration that hints he might invoke the so-called "nuclear option," a rule change that would essentially blow up the filibuster by allowing for a simple majority of 51 votes to confirm a nominee.

Trump has said he favours that option if his nominee is obstructed.

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The Supreme Court building is seen in Washington on June 30, 2014. (Pablo Martinez Monsivais/Associated Press)