Supreme Court choice Neil Gorsuch draws Democrat opposition
If confirmed by the Senate, Judge Gorsuch, 49, would restore the court's conservative 5-4 majority, lost when Justice Scalia died.
The court has the final legal word on many of the most sensitive US issues, including abortion, gender rights and gun control.
House Democratic Leader Nancy Pelosi called Mr Trump's nominee "a very hostile appointment" and "a very bad decision, well outside the mainstream of American legal thought".
Former Democrat presidential contender Bernie Sanders said Judge Gorsuch "must explain his hostility to women's rights, support of corporations over workers and opposition to campaign finance reform".
Republicans would cry foul over a concerted effort to block Judge Gorsuch's confirmation, but it was the Republicans who blocked Barack Obama's nomination for the seat until Mr Obama left office.
Justice Scalia's seat became available 10 months before the end of Mr Obama's presidency, but Republicans refused even to debate his pick of Judge Merrick Garland, claiming it was too close to an election.
Even if Judge Gorsuch makes it through the Senate Judiciary Committee, he will face challenges when the entire chamber convenes for a final vote.
Democrats may seek to prevent that second vote by prolonging, or filibustering, the debate. In that case, the nomination would need 60 votes rather than a simple majority.
With Republicans only holding 52 Senate seats, they may have to change Senate rules in order to approve Mr Trump's nominee.
Abortion: He has not spoken out about Roe v Wade, the case which legalised abortion nationwide in 1973, making in difficult to pin down where he stands on the issue.
Birth control: Judge Gorsuch has supported religious institutions which objected to requirements for employers to provide access to contraception. In one of his most high-profile cases, he defended the religious owners of retailer Hobby Lobby who refused to fund birth control via staff health insurance.
Gun rights: He hasn't ruled directly on firearms restrictions, but is thought to be generally pro-second amendment. He once wrote in a legal opinion that a citizen's right to bear arms "must not be infringed lightly".
Euthanasia: He has been vocal about assisted dying, writing a book in 2009 which opposed legalisation.
The highest court in the US is often the ultimate arbiter on highly contentious laws, disputes between states and the federal government, and final appeals to stay executions.
It hears fewer than 100 cases a year and the key announcements are made in June.
Each of the nine justices serves a lifetime appointment after being nominated by the president and approved by the Senate.
The court already has cases this term on the rights of transgender students, gerrymandered voting districts and on the Texas death penalty determination.
It is also likely the court will hear cases on voter rights, abortion, racial bias in policing and US immigration policy, and possibly on Mr Trump's controversial executive order banning refugees.
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