Vice President Dick Cheney warned Wednesday that despite the passage of more than four years since Sept. 11, 2001, the threat of terrorism inside the United States remains serious.

Cheney added that strong measures taken by the Bush administration are one reason no more attacks have occurred, and defended the president's use of electronic surveillance to listen in on terrorists.

"There are no communications more important to the safety of the United States than those related to Al Qaeda that have one end in the United States," Cheney said.

Click into the video tab to the right to watch a report by FOX News' Jim Angle.

The vice president argued that had the United States been able to track communications like this before Sept. 11, officials might have picked up overseas calls of two of the hijackers who flew into the Pentagon.

Cheney said as time passes and Sept. 11 becomes more distant a memory, it's a natural impulse to play down the threats faced from terrorism.

"Obviously, no one can guarantee that we won't be hit again. But neither should anyone say that the relative safety of the last four years was an accident. America has been protected not by luck but by sensible policy decisions," Cheney said.

Cheney was making reference to the president's decision, among other things, to authorize more vigorous eavesdropping, which Bush described over the weekend in very simple terms.

"If somebody from Al Qaeda is calling you, we'd like to know why," the president said.

But Sen. Russ Feingold, D-Wis., told reporters on a conference call Wednesday that the president does not have the power to allow the kind of domestic surveillance he has authorized.

"What we're not open to is the president deciding on his own that he ought to make up a law because he's frustrated that he can't do whatever he wants. That is unacceptable," said Feingold, who voted against the Patriot Act in 2001.

Feingold argued that in both debate over the Patriot Act and the controversy over eavesdropping by the National Security Administration, the administration pays too little attention to civil liberties.

"They are just saying it has to be exactly their way and showing essentially no sensitivity to the freedom and civil liberties issues that members of both parties have raised in the Senate," he said.

But Cheney insists the program is legal and noted that he served in the Ford administration just after President Nixon was forced to resign in part for eavesdropping on political opponents.

Cheney said Ford and others worked hard to protect civil liberties and that Bush is just as concerned.

"He has made clear from the outset, both publicly and privately, that our duty to uphold the law of the land admits no exceptions in wartime. The president himself put it best: He said, 'We are in a fight for our principles and our first responsibility is to live by them,'" he said.

But even before Bush authorized the new program, then-NSA chief Michael Hayden expanded surveillance of terrorists.

Just three weeks after the attacks, he asked to brief the House Intelligence Committee, and according to letters by House Minority Leader Nancy Pelosi, D-Calif., that she got declassified, Hayden told lawmakers he was taking "an expansive view" of his authority to conduct electronic surveillance. Pelosi raised questions about that position in her letters.

"I am concerned whether, and to what extent, the National Security Agency has received specific presidential authorization for the operations you are conducting," she wrote.

Hayden sent a letter to Pelosi 10 days later apologizing for any confusion and saying he was attempting to emphasize that he had "used his authorities to adjust NSA's collection and reporting."

A short time after the letter, the president did give the NSA authorization to expand further its monitoring of terrorist communications. Pelosi expressed concerns about that, too.

Now, Rep. Jane Harman, D-Calif., the ranking Democrat on the House Intelligence Committee, is telling the White House that it is obligated under the law to brief the entire committee on the new program, which is already likely to be the subject of one or more hearings.

Click into the video tab above to watch a report by FOX News' Jim Angle.