At a crucial moment during diplomacy over North Korea's nuclear ambitions this year, Vice President Cheney late one night persuaded President Bush to draft new, more hard-edged instructions for U.S. negotiators in Beijing -- which Secretary of State Colin L. Powell only learned about the next day.

And during negotiations on last year's tax-cut bill, when House and Senate Republican lawmakers were barely on speaking terms, it was Cheney who went up to Capitol Hill and in a series of closed-door meetings cut through animosity and arranged the deal that passed the Senate by a single vote -- his own.

Cheney, who tonight debates his Democratic challenger, Sen. John Edwards (N.C.), is arguably the most powerful vice president in U.S. history. He is also the administration's essential man.

He roams across the foreign and domestic policy landscape, identifying issues on which he can make a difference. When he chooses to insert himself into the process, he is a powerful force for resolving problems -- or an unmovable roadblock that thwarts the agenda of others, especially Powell.

"He has become the national security adviser," said David Rothkopf, a visiting scholar at the Carnegie Endowment for International Peace, who interviewed more than two dozen administration officials for an upcoming history of the National Security Council. "Time after time, he has co-opted the leadership and policy-shaping role that the national security adviser or secretary of state usually has."

But Rothkopf said assertions by Cheney's detractors that he is a secretive puppetmaster, wreaking havoc on administration policy, are overblown. "He is not a monster. He is not Darth Vader," he said. "He is a very purposeful, thoughtful guy, but highly conservative to the point of being ideological."

Although Cheney's impact is felt, he leaves few fingerprints, administration officials say. He often doesn't tell his staff what he thinks, or even what he did in private meetings with lawmakers or the president. He also chooses his moments carefully: He may skip repeated meetings of senior officials on a particular issue, appearing only when his intervention could make a difference.

"He was kind of a one-way street. You give him information, but you don't know what he does with it," said Cesar V. Conda, for three years his top domestic policy aide. "He is the quintessential policy wonk. We would give him three-inch-thick binders full of information, and he would read every single page, every fact and figure."

Lawrence B. Lindsey, director of the National Economic Council in the first half of Bush's term, said the outside image of Cheney as a Machiavellian figure inside the administration may stem in part from the way he absorbs information.

"What you see is what you get. He's a very substantive person," Lindsey said. "He is not the most expressive person; it's a real midwestern-strength kind of thing. You get the sense of him taking it all in without signaling what he thinks. Then he makes up his mind and results happen."

Cheney's small, effective and intensely loyal staff also works to provide him with intelligence on key issues bubbling through the bureaucracy. Often, in tandem with allies such as the Defense Department, they may push an issue in a certain direction, making Cheney's personal involvement unnecessary. But if that fails, he still has a chance to swing the debate in meetings with Bush's top advisers -- or later, in a private conversation with the president.

Cheney's resume includes stints as White House chief of staff, House minority whip and defense secretary. He "has a great sense of how things work in Washington," said an official who has worked for Cheney in the administration of President George H.W. Bush and in the current administration. "He loves to hear tales of bureaucratic infighting," the official added. Cheney then uses the information gleaned from those lower-level conflicts to inform his strategy for meetings with other top officials.

Though Cheney's immediate staff is relatively small, the vice president has allies sprinkled throughout the administration. Rothkopf said Cheney has "an unprecedented virtual staff, especially in Defense, which he has worked very effectively."

Cheney's aides say this is overstated. But one administration official, who often agrees with Cheney on the issues, described the vice president as a "trump card" in the tense interagency conflicts that have characterized this administration. "You can only play him selectively and only that often," he said.

Cheney doesn't win every battle. During the investigation by the Sept. 11 commission, he opposed releasing a crucial intelligence briefing -- warning al Qaeda planned attacks in the United States -- that Bush received a month before the attacks. Cheney believed release of the information would chill intelligence reporting. But White House political advisers won that debate and the document was released to the media.

Following are two case studies of how Cheney operates in the arenas of domestic and foreign policy. Given the power and influence of the vice president, few officials involved agreed to be quoted by name. The vice president declined a request for an interview.

Cutting a Tax Deal

In the last week of May 2003, Bush had a problem. He had spent months trying to get his third tax cut through Congress, and needed a deal by Memorial Day in order to get tax rebate checks to the voters by July. But House and Senate Republicans were split over the size and shape of the bill.

So he sent Cheney to Capitol Hill to find a solution.

When the bill was proposed at the start of 2003 as a $726 billion package, Cheney had already had a significant impact. He convinced the president that the bill needed to accelerate cuts in individual tax rates, overriding concerns of White House political advisers that cuts in rates for wealthy taxpayers would fuel attacks by Democrats.

Now, the key problem was that the influential chairman of the Senate Finance Committee, Charles E. Grassley (R-Iowa), had made a commitment to two senators that the tax bill would not be larger than $350 billion over 10 years. But there was a dispute over what the agreement meant.

Grassley thought the promise covered only tax cuts; one of the senators, George Voinovich (R-Ohio), believed the $350 billion figure also included spending that would add to the deficit -- $20 billion in financial aid to states that the administration already was not happy to see attached to its tax bill. The unresolved dispute meant the tax cut might have to be reduced to at least $330 billion.

House leaders could not believe they would have to scale back their ambitions further because of a vague promise made to a senator. Rep. Bill Thomas (R-Calif.), chairman of the Ways and Means Committee, fumed to Grassley that a deal was a deal, and the whole thing was an outrage. He stormed out of one meeting in anger.

Enter Cheney. The vice president first determined that Voinovich was not going to change his mind. He met privately with Thomas, before bringing Voinovich into the meeting to make it clear to Thomas that the Ohio Republican would vote against the bill unless the House compromised.

Cheney held most of these sessions with no staff members in the room -- not even his own. Candida Perotti Wolff, his legislative aide, stood outside as Cheney cut the deal, and then never received a report from him about what was discussed. "He prefers to do these meetings without staff, and he does not tend to divulge what happens behind closed doors," she said.

One congressional official who attended some meetings between Cheney and the lawmakers said he was astonished at the level of detail the vice president understood about the complex issues under discussion, particularly as lawmakers had to reshape the tax bill to fit into the smaller $330 billion box.

"He was very calm, very deliberate. He had an air of confidence and control over the details," the official said. Within two days, Cheney had resolved the impasse. The tax bill passed the Senate, 51 to 50, with Cheney casting the tiebreaker vote in his role as president of the Senate.

Steering North Korea Policy

Few issues have divided the Bush foreign policy team as much as North Korea's nuclear ambitions. Cheney has played a critical -- but largely unknown -- role in this debate.

On one side are those, such as Powell, who think North Korea can be persuaded to abandon its nuclear ambitions in exchange for eventual aid, guarantees of security and other diplomatic incentives. Other top officials, such as Cheney, profoundly mistrust the North Koreans and think political and economic pressure will be necessary to force Pyongyang to capitulate. Bush has given rhetorical support to the first approach but has never settled the question internally.

Cheney has tipped the scales at key moments. Administration officials had agreed the United States would not be drawn into one-to-one negotiations with North Korea unless other countries from the region were at the table. North Korea, the United States and four other countries finally met in Beijing in August 2003.

But the results were minimal and, by December, China was pressing for another meeting that they hoped would demonstrate progress. Chinese officials drafted a statement to be released at the end of the meeting, saying quick approval would ensure that the North Koreans would attend a round of talks in December.

When Bush's top foreign policy aides met on Dec. 12 to discuss the Chinese initiative, Cheney -- who had not attended a North Korea meeting for months -- unexpectedly showed up. The text, as drafted by the Chinese, did not call for "irreversible" dismantling of North Korea's programs or mention "verification," two key phrases in previous U.S. statements.

The vice president weighed in strongly at the meeting, saying those phrases needed to be in the statement. At one point, Cheney invoked the president and said he had made it clear that the United States doesn't negotiate with tyranny, according to two officials who attended the meeting. "We don't negotiate with evil; we defeat it," Cheney declared. That ended the discussion: The Chinese draft was rejected.

Kevin Kellems, Cheney's spokesman, expressed doubt that Cheney put it that bluntly: "I don't think he said it. I have never heard him say anything like it."

When the six-nation talks in Beijing finally resumed in February, Cheney intervened again. On Thursday evening, Feb. 26 -- Friday morning in China -- Cheney met with Bush and persuaded him to send new instructions to the U.S. delegation as it sat down for another day of discussions.

The original instructions to the delegation said that any joint statement issued after the talks must include language on a "complete, verifiable irreversible dismantlement" of North Korea's weapons, known in the diplomatic world by the shorthand of "CVID."

But the North Koreans had rejected that during the talks. The chief U.S. negotiator, Assistant Secretary of State James A. Kelly, reported back about the impasse, wondering if the delegation instead should try to obtain a bland diplomatic statement short of original U.S. goals.

With Powell and his deputy, Richard L. Armitage, away at a black-tie event, Bush drafted new instructions with Cheney's input. The instructions -- which in diplomatic terms suggested the administration's "continued support" of the six-nation talks was in jeopardy unless the U.S. demands were met -- were dictated over the phone by White House official Michael Green to the delegation, bypassing the standard State Department cable system.

Powell and Armitage did not find out about the new instructions until they woke up next morning, and Powell began fielding anguished calls from his Asian counterparts. The Chinese foreign minister told Powell the North Koreans were now willing to sign a more generic statement calling for continued talks.

That afternoon, Powell pulled Bush aside before a luncheon with German Chancellor Gerhard Schroeder, and argued for accepting a statement that the U.S. government 12 hours earlier said was unacceptable.

Bush reluctantly agreed -- but, despite Chinese assurances, the North Koreans rejected it anyway.

Vice President Cheney can be a driving force -- or a roadblock.One observer of Vice President Cheney calls him as "a very purposeful, thoughtful guy, but highly conservative to the point of being ideological."