It concluded that while there was no evidence that any votes were changed in actual voting machines, “Russian cyberactors were in a position to delete or change voter data” in the Illinois voter database. The committee found no evidence that they did so.

In his testimony to two House committees on Wednesday, Mr. Mueller had sought to highlight the continued threat that Russia or other adversaries would seek to interfere in the 2020 elections. He said many more “countries are developing capability to replicate what the Russians have done.”

While the Senate Intelligence Committee’s findings were bipartisan, they came on a day when Senator Mitch McConnell, Republican of Kentucky and the majority leader, moved again to block consideration of election security legislation put forward by Democrats.

Mr. McConnell has long opposed giving the federal government a greater hand in an institution of American democracy typically run by the states.

While the report is not directly critical of either American intelligence agencies or the states, it described what amounted to a cascading intelligence failure, in which the scope of the Russian effort was underestimated, warnings to the states were too muted, and state officials either underreacted or, in some cases, resisted federal efforts to offer help.

Even today, after a two-and-a-half-year investigation, the committee conceded that “Russian intentions regarding U.S. election infrastructure remain unclear.” Moscow’s intelligence agencies — chiefly the G.R.U., Russia’s main military intelligence unit — may have “intended to exploit vulnerabilities in the election infrastructure during the 2016 elections and, for unknown reasons, decided not to execute those options.”

But more ominously, the report suggested that it might have been cataloging options “for use at a later date” — a possibility that officials of the National Security Agency, the Department of Homeland Security and the F.B.I. said was their biggest worry.

That was of concern to the committee because testimony about election machines, which are disconnected from the internet, suggested the most efficient way to alter votes was with physical access to the machines or computers rather than programming them with ballots.

Given the potential for further incursions into the election system, the move by the intelligence agencies to redact large portions of the public version of the report touched off behind-the-scenes battles with members of the committee.

The deletions were so substantial that even the committee’s recommendations for the future were not spared: The section heading on the final recommendation read “Build a Credible,” but the remainder of the heading, and two paragraphs that follow, were blacked out.

But the solutions appear distant. Some states, like New Jersey, appear not to have the money to fix a voting machine infrastructure that has no paper backup to its balloting process, making a truly reliable audit impossible.

Other states still have highly vulnerable registration databases, federal officials say. Those vulnerabilities are so sensitive that the Intelligence Committee did not reveal by name which states were the most heavily compromised — referring to the states only by number to protect their identities.

In one case study, titled, “Russia Access to Election Infrastructure: State 2,” the only unredacted line reads, “Separately, G.R.U. cyberactor breached election infrastructure in State 2,” with all details eliminated.

Mr. Burr argued that the Department of Homeland Security and state and local elections officials had since “dramatically changed how they approach election security,” showing progress that served as “a testament to what we can accomplish when we give people the opportunity to be part of a solution.”

But Senator Ron Wyden, Democrat of Oregon, appended an impassioned dissent to the report, arguing that the committee did not go far enough. “The committee report describes a range of cybersecurity measures needed to protect voter registration databases,” he wrote, “yet there are currently no mandatory rules that require states to implement even minimum cybersecurity measures. There are not even any voluntary federal standards.”

The committee found that the Department of Homeland Security and the F.B.I. warned states in the late summer and fall of 2016 of the threat of Russian interference. But they did not provide election officials with “a clear reason” to take the threat more seriously than other warnings that are regularly issued, the report said.

The first public warning was issued Oct. 7, 2016, but within an hour, it was washed away by the revelation of a tape in which Donald J. Trump was heard making comments about how he would grab women, and by the release by WikiLeaks of excerpts from emails hacked from the account of John D. Podesta, Hillary Clinton’s campaign chairman.

Even at that time, the report makes clear, the agencies did not understand the scope of the Russian effort. It noted that Michael Daniel, President Barack Obama’s cybersecurity coordinator, had been convinced that the Russians had gone after all 50 states — because they are thorough. But it was only two years later that official intelligence assessments concluded that he was right.

Mr. Daniel’s position at the White House has since been eliminated by John R. Bolton, the national security adviser.

While the report praised the steps the agencies have since taken to assist in securing elections, the committee found that concerns about aging voting equipment remain.

“As states look to replace machines that are now out of date, they should purchase more secure voting machines. At a minimum, any machine purchased going forward should have a voter-verified paper trail,” a summary of the report said, while adding that “states should remain firmly in the lead on running elections.”

The states say they do not have the money to conduct a replacement program by November 2020.