Scheduled to testify under subpoena at a Senate hearing on Tuesday, former Enron Chairman Kenneth Lay will assert his right against self-incrimination and refuse to answer questions, his spokeswoman said Sunday night.

"Under the instruction of counsel, Mr. Lay will exercise his Fifth Amendment rights at the Tuesday hearing," Kelly Kimberly said in Houston.

She declined further comment.

Two committees snubbed by Lay a week ago have issued subpoenas compelling him to appear, the first grilling coming Tuesday before the Senate Commerce Committee. Some lawmakers had said they expected he would assert his constitutional right against self-incrimination, though his attorney had not previously indicated he would do so.

Lay has also been subpoenaed to appear Thursday at a hearing of the House Financial Services subcommittee on capital markets.

Lawmakers say they have not considered granting immunity from prosecution to Lay or other Enron executives in return for their testimony, because they do not want to interfere with the Justice Department's criminal investigation of Enron. Congress can compel witnesses to show up but cannot force them to answer potentially incriminating questions without granting them immunity from criminal prosecution.

Lay's colleague at Enron, former chief executive Jeffrey Skilling, did testify last week.

But a number of leaders of Congress' investigations of the Enron collapse made clear earlier Sunday that they didn't believe the sworn testimony of Skilling. One suggested Skilling could face accusations of perjury as a result of his testimony.

Lay, a political backer of President Bush, has not spoken publicly about the Enron disaster since the company entered bankruptcy in December.

Lay's wife, Linda, said recently there were some things about Enron's finances her husband wasn't told about by other company officials.

After an intense week of hearings, lawmakers say they have strong evidence of illegal activity surrounding the failure of the energy-trading company, which slid into the biggest bankruptcy in U.S. history on Dec. 2.

A House investigative panel heard hours of conflicting testimony Thursday from Skilling and other top company officials. Skilling said he knew few details of the complex web of partnerships that brought down Enron, and he insisted he was never warned of problems with the arrangements.

Rep. Jim Greenwood, R-Pa., chairman of the House Energy and Commerce subcommittee, and Rep. Billy Tauzin, R-La., who heads the full committee, told CBS' Face the Nation that they didn't believe Skilling.

"He was totally incredible," said Tauzin. "This is the guy who was in charge of the corporation. ... I'm afraid he may have put himself in some legal jeopardy as a result."

Asked whether Skilling could face a perjury indictment by federal prosecutors, Tauzin replied, "That could happen. I mean you can't come to Congress ... take that oath as he did in front of Jim Greenwood, and then not tell the truth."

Tauzin said: "He could have some real problems."

Skilling's attorney, Bruce Hiler, said later he was "shocked at the unsupported charges being leveled at our client."

Tauzin and Greenwood cited instances in which Skilling said he couldn't recall details of key conversations that subordinates testified they had with him concerning Enron's finances. In one, Enron's new president and chief operating officer, Jeffrey McMahon, said he was transferred out of his job as treasurer shortly after he complained to Skilling about the partnerships in a 30-minute meeting in March 2000.

Some Democrats highlighted the political dimensions of the Enron collapse. Lawmakers of both parties said they hoped the debacle would give a boost to legislation before the House this week designed to curb money's influence on federal elections.

"Ken Lay is the poster child for cash-and-carry government," Senate Commerce Committee Chairman Ernest Hollings, D-S.C., said on CBS' Face the Nation.

He cited Enron's ties with several Bush administration officials and its donations to Bush's presidential campaigns as well as those of numerous senators and House members from both parties.

As the Justice Department and the Securities and Exchange Commission investigate Enron and its longtime auditor, the Arthur Andersen accounting firm, politicians in both parties have scrambled to return campaign contributions connected to Enron and its executives.

The Associated Press contributed to this report.