ASHINGTON, Dec. 1 — The issue of civil liberties has created a classic balance- of-power struggle between Senator Patrick J. Leahy, the Democratic chairman of the Senate Judiciary Committee, and Attorney General John Ashcroft, a onetime colleague now leading the administration's domestic war on terrorism.

In a series of testy letters, blunt interviews, chilly phone calls and formal committee hearings, Mr. Leahy has demanded that Mr. Ashcroft explain and defend the sweeping police, detention and prosecutorial powers assumed by the administration in recent weeks.

In an interview, Mr. Leahy, a former prosecutor from Vermont and a 25-year- veteran of the Senate, said: "I don't want a struggle. First and foremost, as an American and as a Vermonter, I want to see us protected from terrorism. But I want it done in a way that does not diminish the basic protections of the Constitution."

The administration and its allies argue that it has maintained that balance, and note that the public is overwhelmingly supportive of its approach, judging from the public opinion polls. And they dispute the idea that Mr. Ashcroft has failed to adequately consult with Congress.

Mindy Tucker, a spokeswoman for Mr. Ashcroft, said, "The administration has consulted quite a bit with Congress." She added, though, "There are times when the attorney general exercises power that has already been legislated to him, by Congress, and during those times he may or may not consult with them ahead of time."

But this dispute, which will be fully joined when Mr. Ashcroft himself appears before the committee next week, is not just an abstract clash of executive power and legislative prerogative. It also has a personal and political subtext.

During the second term of the Clinton administration, Mr. Ashcroft was a deeply conservative member of the Senate Judiciary Committee, regularly squaring off with ideological opposites like Mr. Leahy on issues like judicial nominations.

Mr. Leahy says he and Mr. Ashcroft had a collegial relationship during their time on the committee and worked together on issues like privacy. But when Mr. Ashcroft was nominated to be attorney general early this year, Mr. Leahy opposed him in the bruising confirmation battle that followed, arguing that Mr. Ashcroft was simply too divisive for the job.

Mr. Leahy says that after Mr. Ashcroft took office, the two men had a reconciliation.

"I told him after he was confirmed, even though I voted against him, I told both him and the president that as far as I was concerned, he was now our attorney general," Mr. Leahy said. "I was starting with a blank slate, and that as ranking member, then as chairman of the committee, I would do the best I could to help him be the best attorney general possible."

But the era of good feelings was brief; tensions have grown since the terrorist attacks.

In the negotiations between Mr. Leahy and the administration over an antiterrorism bill, Mr. Ashcroft appeared at a Republican news conference to denounce the Democrats for moving too slowly on legislation to give the administration new powers for the investigation, surveillance and detention of suspected terrorists.

Mr. Leahy ultimately agreed to a bill that granted the administration much of what it wanted, much to the dismay of civil libertarians. In the view of some liberals, Mr. Leahy had compromised far too much, encouraged by Democratic leaders like Senator Tom Daschle of South Dakota, the majority leader, who had little interest in confronting the administration on civil liberties.

Then, in a matter of days, the administration and Mr. Ashcroft began a series of unilateral actions to expand their powers even more, which provoked Mr. Leahy to protest, strenuously.

In a letter to Mr. Ashcroft on Nov. 9, Mr. Leahy declared, "I have felt a growing concern that the trust and cooperation Congress provided is proving to be a one-way street." In a signal-sending appearance on the NBC News program "Meet the Press," Mr. Leahy was asked if he was upset with Mr. Ashcroft and bluntly replied, "Yes, very much so."

What was particularly grating, several senators on the Judiciary Committee said, was learning of the administration's actions only through the news media. Senator Arlen Specter, Republican of Pennsylvania and a member of the Judiciary Committee, said in an interview, "The point is, the Judiciary Committee — Leahy — did give the administration what it wanted." He added, "So it's not exactly an uncooperative or halting Congress."

Others dispute the notion that this is a clash of institutions. "I don't see this as a battle between Congress and the president," said Richard A. Samp, chief counsel for the conservative Washington Legal Foundation.

At the committee's first hearing this week, Mr. Samp said, "With the possible exception of Senator Specter, it sounded like all the Republicans were on the side of the administration, and the Democrats were the ones raising questions."

Mr. Samp argued that the dispute was fundamentally a partisan clash between Democrats, "who are more naturally inclined to be concerned with civil liberties in a criminal context," and Republicans, who are much less so.

Mr. Leahy sees it differently. In an interview, after the administration's executive order allowing the creation of military tribunals to try foreigners accused of terrorism, he complained: "There's been no consultation. These things just get announced: `George Washington got a British spy once by doing this, so thank goodness we've got recent precedents.' "

Mr. Leahy added, "Maybe part of this is the hubris of, you're riding high in the polls and you feel you can operate by fiat."'

With the specter of critical Senate hearings led by Mr. Leahy looming, Mr. Ashcroft seemed to be moving in a conciliatory direction this week. "I think it's entirely proper that the United States Senate and House exercise oversight over the Justice Department," he told reporters. "I have the highest level of respect and regard for these elected representatives of the people."

But the political and partisan currents are strong. In the hearings this week, Senator Orrin Hatch, Republican of Utah, rallied firmly to the administration's defense, declaring, "I, for one, believe that the steps taken by our law enforcement and intelligence communities have saved us from even more harm."

The public opinion polls show voters are generally supportive of the administration's actions on military tribunals and the detention and questioning of Middle Eastern immigrants. The risks in bucking that tide, some say, are substantial.

Referring to Mr. Leahy, one Republican leadership aide in the Senate said, "During wartime, when so many have been killed, there seems to be a disconnect between what's going on in this country, and what he's doing in his committee."

The Judiciary Committee itself is a politically dangerous place, riven for years by ideological and partisan lines. During the Clinton administration, when Republicans controlled the Senate, Democrats complained bitterly that the Republicans — particularly conservatives like Mr. Ashcroft — were blocking his judicial nominations. Now, Mr. Leahy is drawing increasing fire from conservatives who accuse him of "judicial obstruction" of Mr. Bush's nominees.

Still, on the civil liberties issue, Mr. Leahy does have some bipartisan political cover from lawmakers like Mr. Specter. And he will be joined in the spotlight next week by other Democrats, including Charles E. Schumer of New York, who will act as chairman for a hearing on Tuesday on military tribunals.

The challenge facing Mr. Leahy is how to scrutinize the administration's record on civil liberties while assuring the public that national security remains paramount. "Nobody up here is for crime," Mr. Leahy said carefully in an interview. "Nobody is for terrorists. But let's work together to find the right tools."