Kennedy diagnosed with malignant brain tumor

"We hope and pray that they will be able to treat it and that he will experience a full recovery," said presumptive Republican presidential nominee Sen. John McCain on his "Straight Talk Express."

"I have described Ted Kennedy as the last lion in the Senate," said McCain. "And I have held that view because he remains the single most effective member of the Senate."

Democratic presidential candidate Sen. Hillary Clinton offered her own well wishes to Kennedy, saying his "...courage and resolve are unmatched, and they have made him one of the greatest legislators in Senate history."

Her rival for the nomination, Sen. Barack Obama, called Kennedy a "fighter for his entire life," and said, "I have no doubt that he will fight as hard as he can to get through this."

And in a statement issued by the White House, President Bush also expressed his concern for the ailing Senator.

"Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit...We join our fellow Americans in praying for his full recovery."

The 'liberal lion'
The second-longest serving member of the Senate and a dominant figure in national Democratic Party politics, Kennedy was elected in 1962, filling out the term won by his brother John

Video   McCaskill on senator May 20: NBC's Andrea Mitchell discusses the Kennedy diagnosis with Sen. Claire McCaskill, D-Mo. MSNBC

His eldest brother, Joseph, was killed in a World War II airplane crash. President John Kennedy was assassinated in 1963 and his brother Robert was assassinated in 1968.

Kennedy is active for his age, maintaining an aggressive schedule on Capitol Hill and across Massachusetts. He has made several campaign appearances for the Illinois senator in February, and most recently another in April.

He is the senior senator from Massachusetts, the Senate's second-longest serving member, and is not up for election again until 2012.

Were he to resign or die in office, state law requires a special election for the seat no sooner than 145 days and no later than 160 days after the vacancy occurs.

The law was changed in 2004, when Sen. John Kerry of Massachusetts became the Democratic presidential nominee and Republican Mitt Romney was the state's governor. Prior to the change, the governor would have appointed a replacement who would have served until the next general election.

That would created the opportunity to install a fellow Republican in office, something lawmakers in the vastly Democratic state wanted to avoid.

Among the potential candidates for a Senate vacancy would be Democrats Martha Coakley, the state's attorney general; Rep. Edward J. Markey, former U.S. Rep. Joseph P. Kennedy II and Kennedy's wife, Vicki.

Among the potential Republican candidates could be Romney or former Lt. Gov. Kerry Healey.

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