Democrats are increasingly antsy over the lineup President-elect Donald Trump has announced for his Cabinet, but they are also powerless to do very much to stop him — thanks to their own leader, Sen. Harry Reid.

It’s unlikely Mr. Reid had a President Trump in mind when he pulled the so-called “nuclear option” trigger three years ago, changing Senate rules to eviscerate the use of a filibuster to block presidential nominees.

Set up to help President Obama’s picks overcome GOP opposition, those same rules now will clear the path for Sen. Jeff Sessions to become the new attorney general, Rep. Tom Price to head Health and Human Services and two billionaires to lead the Treasury and Commerce departments.

Liberal interest groups are pleading with Democrats to fight the nominees, even though there’s not a lot they can do without the filibuster and against a unified GOP.

Despite that, those who helped Mr. Reid said they have no regrets.

“I supported what was done,” said Sen. Ron Wyden, Oregon Democrat. “I think it was in the public interest then. I think it is still in the public interest.”

The Constitution gives the president the power to nominate top officials and the Senate the role of providing “advice and consent.” That has been understood to mean senators must vote to approve each nominee — and, in recent decades, it has meant a nominee must be able to survive a filibuster.

Both parties trade blame for who’s responsible for things. Democrats, however, intensified the partisan battles under President George W. Bush, using the filibuster to prevent him from naming a series of conservative appeals court judges.

The GOP retaliated under Mr. Obama. Mr. Reid, despite having been part of the Bush-era filibusters, promised to defend the use of the tool as leader. He didn’t. With the support of his fellow Democrats he used a shortcut procedure to change Senate rules: Any nominee save for the Supreme Court can now be confirmed by majority vote.

Under the new rules Mr. Reid managed to push through controversial Obama picks including Alejandro Mayorkas as deputy secretary at Homeland Security and former Rep. Melvin Watt becoming director of the Federal Housing Finance Agency.

The Senate also voted to confirm three judges — Patricia Ann Millett and Cornelia T.L. Pillard in 2013 and Robert Leon Wilkins in 2014 — to the powerful U.S. Court of Appeals for the D.C. Circuit, which hears a number of cases involving federal agencies and has become a proving ground for future Supreme Court nominees.

The bench of the D.C. Circuit is now composed of seven Democrat-appointed justices and four Republican-appointed justices.

“This is not about Democrats versus Republicans,” Mr. Reid said at the time of the rules change. “This is about making Washington work — regardless of who is in the White House or who controls the Senate.”

After Republicans won control of the Senate, the filibuster was less important. They could use other tools, such as control of committees and the calendar, to delay Mr. Obama’s nominees — or use their majority to defeat ones that did come up for a vote.

Democrats had been confident of keeping control of the White House with Hillary Clinton and of taking back control of the Senate. Mr. Reid, who is retiring at the end of this year, even encouraged Democrats to consider expanding his use of the nuclear option to eliminate the 60-vote threshold for Supreme Court nominees.

“It’s clear to me that if the Republicans try to filibuster another circuit court judge, but especially a Supreme Court justice, I’ve told ‘em how and I’ve done it, not just talking about it. I did it in changing the rules of the Senate. It’ll have to be done again,” Mr. Reid told Talking Points Memo.

Now it’s the GOP that is considering that step, with Mr. Trump poised to make a Supreme Court pick and Democrats’ incoming leader, Sen. Charles E. Schumer, threatening a filibuster.

Still, Democrats maintain the decision has not come back to haunt them.

Asked whether he regrets the decision, Sen. Richard Blumenthal, the Connecticut Democrat who has announced he opposes Mr. Price’s nomination, said, “That is the most common question that has been asked of me” since the election.

“The answer is then and now, I believe that majority rule should prevail in the United States Senate,” Mr. Blumenthal said. “That has been my position since my arrival here when I voted to eliminate the filibuster, and the vote of the majority should prevail.”