The 81-year-old Mr Nixon was stricken while preparing for dinner at his New Jersey home on Monday evening, and was rushed to the intensive care unit of the Cornell Medical Centre in nearby New York. A spokeswoman said he was 'alert, awake and attentive'. Last night, Mr Nixon's doctors said he was 'out of grave danger' and no change in his condition was likely in the next few days.
Among the first to visit the hospital were Julie Eisenhower and Tricia Cox, the two daughters of the 37th President, and Mr Nixon's old friend the evangelist Billy Graham. From California, Henry Kissinger, who helped to mould Mr Nixon's foreign policy successes of the early 1970s, said all Americans 'should now recall how much he has done for our country'.
But those successes - the historic 1972 visit to Communist China, improved relations with the Soviet Union and extrication of the US from the Vietnam War - will doubtless be overshadowed by the Watergate scandal, over which Mr Nixon in August 1974 became the first President to resign his office rather than be impeached.
Watergate, shorthand for the bungled burglary attempt at the Democratic party headquarters in June 1972 and the subsequent cover- up, portrayed the dark side of Richard Nixon, suspicious to the point of paranoia and ready to bend the law to his political purposes.
Out of office, he spent the next few years in exile and disgrace in California. But after returning to the East Coast in 1980 to be closer to his family, he re-emerged as an elder statesman on foreign affairs, whose counsel was sought by Presidents Reagan, Bush and Clinton. Mr Clinton spoke yesterday with Dr Graham to pass on his concerns, a White House spokeswoman said.
This rehabilitation may prove the final chapter of a political career that has seen peaks and valleys as few others. President Eisenhower's Vice-President from 1953 to 1961, he lost the 1960 presidential election to John Kennedy, and when he was defeated for the California governorship in 1962, Nixon's political career seemed over.
But in 1968 he recaptured the White House for the Republicans, and four years later routed George McGovern in one of the greatest landslides of presidential history. From early 1973, however, Watergate gradually consumed him. Only a controversial pardon from Gerald Ford saved Mr Nixon from indictment and possible conviction as a common criminal.
Mr Nixon at first spent his retirement in seclusion in San Clemente, California, then moved to the New York area. He built up a private fortune through sales of properties obtained through wealthy backers, royalties from books and appearance fees. He slowly emerged from limbo to regain a measure of esteem as he travelled, wrote and spoke on foreign affairs.
By coincidence President Kennedy's widow, Jacqueline Onassis, who suffers from lymph cancer, is in the same hospital recovering from stomach surgery for a bleeding ulcer.