BOSTON -- Politicians from the current president to would-be presidents, from both sides of the political aisle, are reacting with shock to the news that Sen. Edward Kennedy has a malignant brain tumor.

President George W. Bush said in a statement that Kennedy is a man of "tremendous courage, remarkable strength, and powerful spirit."

Fellow Massachusetts Sen. John Kerry said his friend is "one unbelievable fighter" who is determined to continue to battle for the people of the Bay State and the nation.

"He's helped millions and millions of people, in so many ways, at so many different times, from countless big pieces of legislation in Washington to the most personal of issues. Now, everybody needs to pull together on behalf of Ted," Kerry said. "I know that Ted is determined to fight this because he wants to continue his fight for the people of Massachusetts and he wants to continue to fight for everything that he believes in here in the United States Senate and throughout our country."

Republican presidential candidate John McCain said Kennedy was the most effective member of the Senate and wished him a full recovery.

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

Democratic presidential candidate Barack Obama called Kennedy a "beloved friend" and said he deserves the kind of support he's been giving others for so long.

"Sen. Kennedy has been a fighter for his entire life, and I have no doubt that he will fight as hard as he can to get through this," Obama said.

Hillary Clinton, Obama's rival for the nomination, called Kennedy one of the greatest legislators in Senate history.

"Ted 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 Vicki and we are praying for a quick and full recovery," she said.

Democratic Sen. Robert Byrd of West Virginia broke down in tears on the Senate floor.

"My thoughts and humble prayers are with Sen. Kennedy ... my dear friend Ted," he said. "Thank God for you, Ted."

Republican Sen. John Warner said he was "lost for words" when he heard the news.