WASHINGTON — Republicans were leery of the prospect of an independent commission to investigate an assault that had shaken the nation and exposed dangerous threats, fearful that Democrats would use it to unfairly cast blame and a political shadow on them.

Congress was already conducting its own inquiry, some of them argued, and another investigation was not needed. The commission could be a distraction at a vulnerable time, prompt the disclosure of national secrets or complicate the prosecution of those responsible.

The year was 2001, but the clash 20 years ago over the creation of an independent commission to investigate the Sept. 11 attacks bears unmistakable parallels to the one that is now raging in Congress over forming a similar panel to look into the Jan. 6 assault on the Capitol.

To most Americans, the idea of a blue-ribbon commission to dig into the causes of the Capitol riot and the security and intelligence failures that led to the seat of government being ransacked would probably seem straightforward. But in recent days, it has become clear that, as in the past, devising the legislative and legal framework for such a panel is fraught with political difficulty, particularly in this case, when members of Congress experienced the attack themselves, and some now blame their colleagues for encouraging it.

And this time, given the nature of the breach — an event inspired by President Donald J. Trump’s false claims of a stolen election, which were trumpeted by many Republicans — the findings of a deep investigation could carry heavy political consequences.

The tensions intensified this week, when Speaker Nancy Pelosi floated a proposal for the creation of a special panel. Republican leaders denounced her initial plan, which envisioned a commission made up of seven members appointed by Democrats and four by Republicans.

Senator Mitch McConnell, Republican of Kentucky and the minority leader, called her idea “partisan by design,” and compared it unfavorably with the Sept. 11 commission, which was evenly divided. He also predicted that Democrats would use their influence on the panel to focus mainly on violent acts by Mr. Trump’s supporters — who planned and perpetrated the assault — suggesting that its mandate should be broadened to examine left-wing extremists.

“If Congress is going to attempt some broader analysis of toxic political violence across this country, then in that case, we cannot have artificial cherry-picking of which terrible behavior does and does not deserve scrutiny,” Mr. McConnell said.

Ms. Pelosi fired back on Thursday, saying she was disappointed in Mr. McConnell, who she said had earlier indicated his support for a commission similar to the one established after the Sept. 11 attacks.

She accused Republicans of following the lead of Senator Ron Johnson, Republican of Wisconsin, who suggested this week that the pro-Trump mob on Jan. 6 had actually been a mostly peaceful crowd seeded with a few “provocateurs,” including members of a loosely affiliated group of far-left anti-fascism activists, known as “antifa.” (The F.B.I. has said there is no evidence that antifa supporters had participated in the Capitol rampage.)

“He was taking a page out of the book of Senator Johnson,” Ms. Pelosi said of Mr. McConnell. She added that the crucial aspect of devising the commission was to determine the scope of its work, dismissing the exact makeup of the panel as an “easily negotiated” detail.

“I will do anything to have it be bipartisan,” Ms. Pelosi said.

The independent, bipartisan National Commission on Terrorist Attacks Upon the United States was eventually formed and lauded for its incisive report published in July 2004. But first, there were myriad obstacles to its creation.

“It was hard,” said Senator Richard C. Shelby of Alabama, the top Republican on the Intelligence Committee at the time who backed the independent panel over objections from the George W. Bush administration. He wanted a deeper look even though his own committee had conducted a revealing joint review with its House counterpart. “I thought it needed to be broader,” Mr. Shelby said.

Ms. Pelosi, who was the top Democrat on the House Intelligence Committee at the time, was an early proponent of a special commission to fully investigate the attack. She argued that any congressional review would almost certainly be too narrow and that an inquiry by the same government that had failed to prevent the attack would lack public credibility. Her proposal was rejected by the Republican-led House under pressure from the Bush administration, which feared disclosures of intelligence lapses and other shortcomings that could cost their party politically.

Instead, Congress moved ahead with the joint inquiry by the House and Senate intelligence panels, which revealed a failure by the White House to heed warnings about a looming strike on the United States. But even those leading the inquiry believed an independent commission was needed to break free of congressional constraints.

“One of the benefits of a subsequent round of hearings is that you can avoid those interferences,” said Bob Graham, a Democratic senator from Florida and the chairman of the Intelligence Committee at the time.

Senators Joseph I. Lieberman, Democrat of Connecticut, and John McCain, Republican of Arizona, responding to calls from the families of those killed on Sept. 11, pushed forward with a proposal for an independent panel. They built on a long tradition of the United States taking such steps after shattering events like the attack on Pearl Harbor and the Kennedy assassination. But the plan encountered stiff resistance from the Bush administration, which finally agreed to its creation in late 2002 after one last round of foot dragging.

As the commission began public hearings in the spring of 2003, Ms. Pelosi lamented that it had taken so long but lauded the determination required to make it a reality.

“Through the persistence of a member of this commission, former Congressman Tim Roemer, as well as that of Senators McCain and Lieberman, this body was established and has begun its critical work,” she said then.

In the case of the Jan. 6 assault, Congress this week began its own set of hearings into what went wrong. Some lawmakers privately suggested that their work could be sufficient and that an independent panel would be redundant. And at his confirmation hearing on Monday to be attorney general, Judge Merrick B. Garland warned that he supported the idea of an independent inquiry only as long as it would not derail the prosecution of any of those charged in the assault.

The current Congress is much more polarized than it was in the aftermath of Sept. 11 and the creation of the commission is complicated by the fact that Democrats are highly skeptical of the motives of Republicans. Democrats see some of them as complicit in fueling the attack by spreading falsehoods about the presidential election being stolen and then challenging the electoral vote count on Jan. 6.

On Wednesday, Representative Hakeem Jeffries of New York, the No. 5 Democrat, accused top Republicans of not acting in good faith and setting a “bad tone” by joining the unsuccessful effort to overturn the election results.

“All of that said, Speaker Pelosi still presented the framework to the Republicans, which then, of course, instead of leading to some kind of good-faith conversation from them, they immediately launched into a partisan political attack,” Mr. Jeffries said.

But Republicans have suspicions of their own. Even those who have backed the idea of a commission say they will not accept a proposal they see as giving Democrats the upper hand in determining the course of the commission’s work.

“It has to be independent,” said Senator John Cornyn, Republican of Texas. “This can’t be the Nancy Pelosi commission.”