The United States has "credible" information indicating that the al Qaeda terrorist network is preparing a large-scale attack in the United States aimed at disrupting this year's electoral process, Homeland Security Secretary Tom Ridge said today.

But no specific intelligence has provided a target, date or location for the attack, and the department is not raising its security alert from the current elevated "yellow" level, Ridge said. That level, the middle category of five alert conditions, indicates a "significant risk of terrorist attacks."

In a press conference that he billed as an opportunity to remind the public of the continuing terrorist threat and to outline some of the protective measures being undertaken, Ridge said the government does not have any specific information on a plan to attack this summer's presidential nominating conventions. However, he noted that he has declared the conventions to be "national security events" and said he plans to visit the two sites soon to personally review security measures.

The Democratic Party is holding its convention July 26 to 29 in Boston, and the Republicans have scheduled their convention for Aug. 30 to Sept. 2 in New York.

"Credible reporting now indicates that al Qaeda is moving forward with its plans to carry out a large-scale attack in the United States in an effort to disrupt our democratic process," Ridge said. "We lack precise knowledge about time, place and method of attack. But along with the CIA, FBI and other agencies, we are actively working to gain that knowledge."

Ridge said the Madrid train bombings in March and recent arrests of terrorist suspects in Britain, Italy and Jordan have shown that al Qaeda has the "capability" to carry out such an attack. "And they also hold the mistaken belief that their attacks will have an impact on America's resolve," he said.

While the threat level is not being raised, Ridge said, "we have permanent protections in place that did not exist a year ago." He said that "these protections make it harder for terrorists to attack us."

Among the measures Ridge mentioned was an integrated communication system with national, state and local partners that "allows us to make better decisions more quickly and take actions that deter, detect and defuse terrorist attacks."

Some of the measures involve tracking high-risk trucks and rail shipments and preventing the use of radiological "dirty" bombs through better detection equipment, Ridge said.

He said the security of the nation is being strengthened daily, and he called on Americans to remain on the lookout for suspicious activity.

"Efforts each of you make to be vigilant . . . do make a difference," Ridge said. "Every citizen, using their common sense and their eyes and ears, can support our national effort to stop the terrorists. We know that their vigilance in the long run can make a difference."

In response to a question about the possibility of subway and rail bombings aimed at disrupting this summer's political conventions, Ridge noted that mass transit has been targeted by terrorists in the recent past. He said federal officials are working with mass transit authorities in cities nationwide to improve security.

Ridge made the comments following a closed-door briefing that he and FBI Director Robert S. Mueller III gave to senators today.

Senate Majority Leader Bill Frist (R-Tenn.) said after the briefing that the security threat should be kept in perspective and that there is "no reason for panic or paralysis."

At the same time, Frist said, "The country is at some increased risk between now and the time of the presidential election. It's important for people to be aware of that."

He said law enforcement agencies are attuned to the threat and are actively engaged in "deterrence and prevention."

Sen. Charles E. Schumer (D-N.Y.) told reporters after the session that the information he had heard was worrisome.

At the press conference, Ridge was asked whether, in view of al Qaeda's reported aim of disrupting the U.S. democratic process, the Bush administration was sending a message that voting for Democratic presidential candidate John F. Kerry was tantamount to supporting al Qaeda.

"I think that's a wrong interpretation," Ridge said. "These are not conjectures or mythical statements we are making."