President Trump shakes hands with Judge Neil Gorsuch after announcing him as his U.S. Supreme Court nominee at the White House on Jan. 31, 2017. (Jabin Botsford/The Washington Post)

The U.S. Senate headed for its most heated Supreme Court confirmation hearings in more than a decade on Tuesday, with both parties facing intense pressure to prevail in one of the defining political battles of President Trump’s nascent term.

Neil Gorsuch, Trump’s pick to replace the late Antonin Scalia on the high court, is to visit Capitol Hill on Wednesday. Republicans are hoping to confirm the nominee by early April before a two-week Easter recess, allowing Gorsuch to participate in the final cases of the court’s term ending in June.

But in a sign that Democrats were immediately ramping up resistance, Minority Leader Charles E. Schumer (D-N.Y.) and several colleagues declared that Gorsuch would need to earn at least 60 votes to clear procedural hurdles to earn a final confirmation vote. Republicans hold 52 seats in the Senate.

“The burden is on Judge Neil Gorsuch to prove himself to be within the legal mainstream and, in this new era, willing to vigorously defend the Constitution from abuses of the Executive branch and protect the constitutionally enshrined rights of all Americans,” Schumer said in a statement. “Given his record, I have very serious doubts about Judge Gorsuch’s ability to meet this standard.”

That will put pressure on Republicans, who have been agonizing over whether to change long-standing Senate rules to break Democratic resistance — and who are already feeling the heat from supporters yearning to add a conservative voice to the court for the first time since the presidency of George W. Bush.

Majority Leader Mitch McConnell (R-Ky.) said he hopes senators will give Gorsuch “fair consideration and respect the result of the recent election with an up-or-down vote on his nomination, just like the Senate treated the four first-term nominees of Presidents Clinton and Obama.”

In a separate video message, McConnell gushed about Gorsuch: “The president made an outstanding choice.”

Just 31 current senators were in office in July 2006 when Gorsuch was confirmed without opposition to serve on the U.S. Court of Appeals for the 10th Circuit, which has jurisdiction over all or parts of eight western states.

This time around, Gorsuch will be shepherded across Capitol Hill by former senator Kelly Ayotte of New Hampshire, a moderate Republican who lost reelection in November and will lead a team of veteran GOP political operatives overseeing the months-long confirmation fight.

[Ayotte to lead White House team shepherding Supreme Court nominee]

Gorsuch’s fate rests especially with Democrats including Sen. Jeff Merkley (Ore.), who suggested in recent days that he would try to mount a filibuster as payback to Republicans who blocked former president Barack Obama’s final Supreme Court nominee, Merrick Garland, for almost the entirety of 2016.

“This is a stolen seat,” Merkley said in a statement Tuesday night.

(Peter Stevenson,Gillian Brockell/The Washington Post)

Other Democrats said their consideration of Gorsuch will be tied to Trump’s executive order temporarily barring U.S. entry for foreign nationals from seven majority-Muslim countries and for refugees worldwide — and his decision to fire the acting attorney general, Sally Yates, for refusing to defend the order in court.

“In light of the unconstitutional actions of our new President in just his first week, the Senate owes the American people a thorough and unsparing examination of this nomination,” said Sen. Patrick J. Leahy (D-Vt.), a senior member of the Judiciary Committee, which will consider Gorsuch’s nomination.

Sen. Richard Blumenthal (D-Conn.), another member of the panel, added that he had “deep, serious concerns” about Gorsuch.

“If I conclude that Judge Gorsuch is out of the mainstream, then I will pursue every legal tool available to block his nomination,” Blumenthal said.

By threatening to filibuster, Democrats could be laying a tripwire for Republicans to get rid of it entirely. Just a few years ago, it was Democrats who ended the ability to filibuster presidential appointments — with the crucial exception of Supreme Court nominees. If Merkley and other Democrats follow through on their threat, Republicans will not even have the support to bring up Gorsuch’s name for a vote. It would be just the second time in modern history that a Supreme Court nominee was filibustered.

[On the death of the Senate and its long history as the world’s greatest deliberative body]

That very plausible situation will leave Republicans with two unenviable decisions: Give up on getting Trump’s pick through — or get rid of the filibuster so that a simple majority can approve the nominee.

That could pit McConnell’s devotion to Senate customs and tradition against conservatives eager to see one of their ideological peers win a high court seat.

“You change the rules of the Senate, you are doing something pretty draconian,” Sen. John McCain (R-Ariz.) said Tuesday afternoon.

McConnell could also find himself in Trump’s crosshairs, a fate he has avoided thus far, unlike his House counterpart, Speaker Paul D. Ryan (R-Wis.). The president has said he supports a rules-change maneuver, known on Capitol Hill as “the nuclear option,” to avert a filibuster if Democrats stand firm in resistance.

Some Democrats said they would frame their opposition to Gorsuch as more considered than Republican opposition to Garland.

The Judicial Crisis Network, a conservative nonprofit organization serving as an outside ally to McConnell’s leadership team, launched a $2 million ad campaign in four states that Trump won and where Democratic senators face reelection: North Dakota, Indiana, Missouri and Montana.

Carrie Severino, chief counsel for the Judicial Crisis Network, said her group is targeting those senators because they are seen as some of the most likely to join Republicans in confirming Gorsuch. She said the organization plans to defend Gorsuch and also warn Democrats against joining a campaign of “obstructionism.”

“Those senators are going to have a tough choice before them,” she said.

Progressive groups including Democracy for America, meanwhile, demanded “total opposition to all of Trump’s appointees,” including Gorsuch, until Trump rescinds his travel ban.

The Center for American Progress, a liberal think tank, warned Democrats on Tuesday not to relent.

“The only reason that Trump can name this Supreme Court justice is because of the unconscionable actions by Senate Republicans in refusing to even hold a hearing” for Garland, the group said in a memo to senators. “Rewarding such behavior by confirming a radical nominee should not even be an option for Democrats.”

The back-and-forth will put immediate political pressure on dozens of vulnerable Democrats, including Sen. Jon Tester (Mont.). A close ally of Schumer’s, Tester said he does not agree with the dogmatic standard that Merkley and others are imposing to block any Trump nominee. Tester said he was willing to vote for the nominee if the person met his credentials. “Somebody that knows real America and somebody that knows the Constitution and somebody that knows the gravity of the position — like Merrick Garland,” he said.

“I’ll see who he nominates and then we’ll do our due diligence,” Tester said before Trump announced his nominee.

Sen. Joe Manchin III (D-W.Va.), who represents a state Trump won by more than 40 points, urged his Democratic colleagues “to put partisan process aside and allow the process to proceed.”

Manchin and Tester are part of the class of 25 Democrats up for reelection in 2018, 10 of whom come from states that Trump won.

Republican leaders cheered Trump’s choice and touted the pick in national television interviews from the White House East Room on Tuesday night.

“A guy who’s been [on] a three-day drunk could figure out this guy is one of the most qualified people ever to be at the circuit level. . . . If we blow up the Senate over a man like this, it really would be a shame,” said Sen. Lindsey O. Graham (R-S.C.), an occasional Trump critic.

Democrats had been invited by Trump to attend the White House announcement, but they declined, since they were unaware of who the nominee would be. Sen. Richard J. Durbin (Ill.), the Democratic whip, compared the situation to a reality TV show.

“Tonight seemed like an awkward situation, to walk into a room and they pull the curtain back and say, ‘Here’s your next Supreme Court nominee.’ Now what are you going to do next?” he said.

Paul Kane, Philip Rucker and David Weigel contributed to this report.

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