Democrats confronted a nightmare scenario on Friday as they surveyed the wreckage of major political, policy and legal disasters piled atop one another with elections for control of the White House and Congress less than five months away.

Even as they reeled from President Biden’s poor performance on Thursday night in a make-or-break debate with former President Donald J. Trump, Democrats were slammed anew on Friday by the Supreme Court.

In one far-reaching ruling, the court undercut the government’s longstanding power to regulate health care, public safety and the environment — a core tenet of Democratic orthodoxy. In another, the justices handed down a decision that could make it more difficult to prosecute those who attacked the Capitol on Jan. 6, 2021, and potentially Mr. Trump himself, for their roles in the violent insurrection.

It was a day that encapsulated the party’s worst fears about the coming elections and the rightward tilt of the Supreme Court. And it drove home how Republicans and Mr. Trump are within reach of victory in November — putting them in position to achieve a host of policy objectives vehemently opposed by Democrats — even with a presumptive nominee who is a convicted felon and a party that has been in deep disarray and shown little ability to govern.

“It is a bad day for democracy and a threatening day for the rule of law,” said Senator Richard Blumenthal, Democrat of Connecticut and a senior member of the Judiciary Committee.

The pileup left Democrats worried and despairing, and most did not make the usual effort to sugarcoat Mr. Biden’s woeful debate showing.

“Joe Biden is a very good man,” said Representative Steny H. Hoyer, Democrat of Maryland and a longtime House leader. “He respects people and respects the truth and respects the Constitution. He had a bad night. His opponent in the debate is not a good man.”

Mr. Biden’s halting performance prompted a frenzy of speculation about whether he should or even could be replaced as the nominee, a possibility that many leading Democrats dismissed.

But the Supreme Court rulings hit just as hard, underscoring the stakes of the coming election and how the impact of Mr. Trump’s first term in office is still being felt profoundly. The decisions struck at the heart of Democrats’ view of government, one where those who turn to violence to obstruct the peaceful transfer of power should be punished and one where federal agencies must have the power to interpret federal law and protect the public from corporate misbehavior.

Adding insult to injury, the rulings came at a time when Democrats firmly believe the court has been compromised because of the ties of some conservative justices to billionaire businessmen, as well as their tacit support of the Jan. 6 movement. As far as Democrats are concerned, that bias was exposed by the flying of a flag sympathetic to the Jan. 6 rioters at the Virginia home of Justice Samuel A. Alito Jr. Both Justice Alito and Justice Clarence Thomas, whose wife, Virginia, was involved in efforts to overturn the 2020 election, refused to recuse themselves from the case.

Democrats derided the decision in the landmark government power case as a major step backward and another instance where the conservative-dominated court cavalierly abandoned years of well-established precedent to the detriment of the public.

“Today’s Supreme Court ruling once again shamefully overturns decades of precedent, stripping regulatory authority from federal subject matter experts with substantial experience and handing it over to judges with no previous training or expertise on complex matters,” said Senator Alex Padilla, Democrat of California. “This decision is severely misguided and will move us further from governing decisions that are rooted in facts and science.”

The so-called Chevron doctrine that empowered federal agencies in a 1984 Supreme Court case has long been a target of conservatives who complained that it gave faceless federal bureaucrats — many of them perceived to be liberal — too much authority to determine how federal laws are enforced. It was the origin of what Republicans deride as the “administrative state,” and they hailed the court’s action as a breakthrough.

“The Chevron doctrine has plagued Americans for four decades by granting unelected bureaucrats excessive power to determine, and in many cases invent, congressional intent,” said Senator Kevin Cramer, Republican of North Dakota. “This entire scheme grants outsized control to the so-called experts who are unaccountable to voters and have a vested interest in growing federal control.”

The Supreme Court ruling that prosecutors had overreached in seeking obstruction convictions against those who took part in the assault on the Capitol was another blow to Democrats who believe that Mr. Trump and the far right are trying to rewrite the history of what happened as the mob thronged through the Capitol to prevent the counting of electoral ballots for Mr. Biden.

“Today, the Trump-packed Supreme Court made it easier for Jan. 6 insurrectionists to escape accountability for their crimes,” Representative Katie Porter, Democrat of California, wrote on social media. “I was trapped in the Capitol that day, and the rioters’ intent was clear — to stop Congress from doing our job.”

But many Republicans hailed the finding and downplayed the actions of those charged in the Jan. 6 assault.

“The Department of Justice stretched a law in a way more befitting a banana republic than America,” said Senator Tom Cotton, Republican of Arkansas. “This administration tolerates violent crime and an open border, but they manipulated a statute to imprison grandmas anywhere near the Capitol on Jan. 6.”

Despite the ground seemingly shifting beneath them, Democrats saw one potential benefit — the possibility that voters worried about empowered Republicans and the direction of the Supreme Court could be motivated to vote for Mr. Biden, particularly with the prospect that the next president will have new vacancies to fill on the court.

“If people are really paying attention here,” said Mr. Blumenthal, “alarm bells should be going off at kitchen tables around the country.”