WASHINGTON (CNN) -- Sen. Barack Obama said Tuesday that it is "heartbreaking" to hear that Sen. Edward Kennedy is suffering from a brain tumor, crediting the ailing lawmaker with making it possible for him to run for president.
Sen. Ted Kennedy endorsed Sen. Barack Obama for president this year.
"I would not be sitting here as a presidential candidate were it not for some of the battles he fought as a senator: He battled for voting rights and civil rights when I was a child. I stand on his shoulders," Obama said.
In January, Kennedy added his considerable clout in Democratic circles to Obama's campaign, endorsing a candidate he said "appeals to the hopes of those who still believe in the American dream."
Kennedy's diagnosis was made public Tuesday. He was hospitalized on Saturday.
Obama described the Massachusetts senator as "a great friend, someone who always has a kind word to say, even for his adversaries."
Sen. Hillary Clinton said Tuesday that Kennedy's "courage and resolve are unmatched, and they have made him one of the greatest legislators in Senate history."
"Our thoughts are with him and [his wife,] Vicki, and we are praying for a quick and full recovery," she said. "Obviously, he's a fighter."
She called him the "most effective" senator ever.
Sen. John McCain, an Arizona Republican who is the GOP's presumptive nominee for president, had a similar statement.
"Our thoughts and prayers go out to his family and to him. We hope and pray that they will be able to treat it and that he will experience a full recovery," he said.
"I have described Ted Kennedy as the last lion in the Senate," a tearful McCain said on his campaign bus in Florida. "And I have held that view because he remains the single most effective member of the Senate."