Nearly every Democratic senator eventually complied. Oregon’s Sen. Jeff Merkley vowed to obstruct the president’s nominees purely as “a protest” against Republicans. Sen. Chris Murphy of Connecticut promised to “shatter precedent” to stymie nominations.

The result was unprecedented. As had been tradition, Presidents Clinton and Obama had nearly all of their cabinet confirmed in January. Mr. Clinton had 13 out of 14; Mr. Obama, 11 out of 15. Senate Democrats made sure Mr. Trump had only three.

Because the Democrats abolished the filibuster for confirmation votes in 2013, they couldn’t ultimately block any appointment without Republican support. But nominees faced a bombardment of endless “questions for the record.” Democrats voted en bloc against nominees in committee. They consistently refused to consent to routine floor votes, forcing the use of the cloture procedure, then made sure to use every minute of the 30 hours of postcloture debate to delay the confirmation vote as long as they could.

Senate Democrats slowed down and attempted to block the confirmation of 24 of Mr. Trump’s cabinet and cabinet-rank nominees. None faced similar roadblocks during the first terms of Mr. Clinton, George W. Bush or Mr. Obama.

When lower-level nominees are included, Democrats attempted to block 79 executive-branch appointments in the first two years of Mr. Trump’s administration. His six predecessors combined faced the same obstruction only 18 times in their first two years.

While nominees were confirmed in the end, the Democrats’ actions hamstrung the new administration. Cabinet nominees were stuck in a holding pattern while national and international issues waited. Successive cloture votes and harassment prevented the Senate from doing much of anything else in the early days of the 115th Congress. Obstruction for the sake of obstruction preoccupied the president and the Senate at a time when focus could have and should have been elsewhere.

Mr. Schumer sent a signal on Jan. 31, 2017, the day of that massive protest outside his home, when he voted against the confirmation of Elaine Chao as transportation secretary. Expertly qualified and highly respected by both sides, Ms. Chao had previously served as labor secretary for eight years and had been vetted by the U.S. Senate for other confirmable posts. No reasonable argument was made during Senate hearings to oppose her.

As uncontroversial and experienced as she was, Mr. Schumer voted against her confirmation. She was confirmed 93-6, but Mr. Schumer’s colleagues soon followed his lead. Mindless opposition to Mr. Trump’s nominees became a test of party loyalty. The only goal was to prove how strongly Senate Democrats could resist.

Opposing Trump nominees spilled over into the 2020 Democratic presidential primaries. Some senators running for president were accused of going too easy on Trump nominees. The “hell no” caucus took pride in rejecting anyone and everyone. Mr. Schumer’s New York colleague, Sen. Kirsten Gillibrand, boasted that she had “the best voting record against Trump nominees of anyone else running for president.”

Joe Biden has begun to name the men and women he wants to serve in key positions. The U.S. Senate will have something to say about every one. Don’t expect Senate Republicans to forget how the Democrats treated Mr. Trump’s nominees. Don’t expect the tens of millions of Americans who voted for Mr. Trump in 2016 and 2020 to forget it either.

If Republicans hold the majority in the Senate next year, we will have the ability to reject Mr. Biden’s nominees. Unlike the Democrats, we will use that power responsibly. A Republican Senate will treat mainstream nominees fairly.

Yet Mr. Schumer has blown away the old custom of giving a president wide latitude to pick his team. Those who are out of the mainstream will face a gantlet, not a garden party.

The unprecedented obstruction of the previous administration will have lasting consequences for future administrations. Senate Democrats put in place new standards and norms. They won’t be easily washed away.

Mr. Barrasso of Wyoming is chairman of the Senate Republican Conference.