Washington - Stunned silence swept Tuesday across the US Congress, observers said.

One colleague wept.

Senator Edward "Ted" Kennedy, left-wing icon and champion of the rights of workers and the poor, had been diagnosed with brain cancer.

The announcement sent shockwaves throughout Washington's political establishment, with Republican and Democratic leaders alike paying tribute to his achievements and significance in US political life.

Outside medical experts expressed sobering prospects for recovery from the malignant glioma tumor in the 76-year-old senator's left parietal lobe, a diagnosis confirmed through a biopsy taken after a seizure on Saturday.

But many of Kennedy's colleagues were upbeat, believing that the man who fought so hard for voting rights, civil rights, and rights for the disabled would overcome this challenge, too.

Democrat Nancy Pelosi, speaker of the House of Representatives, said she had "confidence" that Kennedy's "fighting spirit" would help him through.

"He's a strong guy, with a great heart," said Senator Chris Dodd, a fellow Democrat from Connecticut, a neighbouring state to Kennedy's native Massachusetts.

Others were more somber.

Robert Byrd, 90, a Democrat from West Virginia, is the only sitting senator who has served longer than Kennedy's 46 years in the chamber.

Byrd put his head down on his desk on the Senate floor and wept. "Ted, my dear friend, I love you, and I miss you," he sobbed. "Thank God for Ted."

Leading Democratic presidential candidate Barack Obama, an African-American senator who received a strong, early endorsement from Kennedy, said that he expected his colleague to "fight as hard as he can."

"I stand on his shoulders," Obama said. "I would not be sitting here as a presidential candidate had it not been for some of the battles Ted Kennedy has fought."

While Kennedy is often on the left wing of the centre-left Democratic Party, he worked with centre-right Republicans on many major pieces of legislation - including with President George W Bush to pass the "No Child Left Behind" education law in 2001.

Kennedy has since been sharply critical of most Bush administration policies, including the implementation of the education reforms on which they cooperated. Nonetheless, the president issued a statement saying he and his family were praying for Kennedy.

"Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit," Bush said. "Our thoughts are with Senator Kennedy and his family during this difficult period."

Senator John McCain, the presumptive Republican presidential nominee, called Kennedy the "last lion in the Senate" and "the single most effective member of the senate." In television footage from his travelling campaign bus, McCain appeared to be choked with emotion while talking about his colleague.

Senator Hillary Clinton, who was disappointed in January over Kennedy's support of Obama, was also shaken, calling the cancer diagnosis "very difficult" news.

"Obviously, he is a fighter. There's never been anybody like him in the Senate," she said. "He's probably the most effective single senator our country has ever seen."

In fact, Clinton has been mentioned by pundits as a possible successor to Kennedy as the senate's "liberal lioness" if she loses her bid for her party's nomination, as now widely expected.

As the last surviving national political figure from his fabled family, Kennedy endured the assassination of two of his older brothers: president John Kennedy was assassinated in office in 1963, and senator Robert Kennedy was killed by a gunman while running for president in 1968. The oldest of the Kennedy brother, Joseph, was killed in World War II.

John Kerry, the 2004 Democratic presidential nominee and Kennedy's fellow senator from Massachusetts, was visibly stricken by the grave word about his colleague.

"Ted Kennedy and the Kennedy family have faced adversity more times in more instances with more courage and more determination and more grace than most families ever have to face," Kerry said.

"This guy is one unbelievable fighter." (dpa)