WASHINGTON — There's an old story, perhaps apocryphal, about how John F. Kennedy, at the tender political age of 42, stood on the Senate floor and beheld his worthy Democratic colleagues contemplating campaigns for the presidency in 1960 — the likes of Hubert Humphrey, Lyndon Johnson and Stuart Symington.

All three were rated political giants of the day, and the story goes that Kennedy, coolly sizing them up, asked himself: "Why not me?" Answering in the affirmative, he decided to run too.

The other day, another young senator named Barack Obama, age 45, could have looked around the same Senate floor, seen the likes of Hillary Clinton, Joe Biden, John Kerry and Chris Dodd, asked himself the same question and received the same answer. Modesty is not an affliction that often visits these premises.

Kennedy did not arrive at his self-inquiry out of the blue. He had come within a whisker of receiving the Democratic vice-presidential nomination four years earlier, when presidential nominee Adlai E. Stevenson had thrown the selection open to the party convention in Chicago.

In Obama's case, a different kind of spotlight has focused on him since his election to the Senate in 2004, when he said he "absolutely" would serve out his full six-year term, and last Tuesday when he announced an exploratory committee as a prelude to a presidential candidacy.

With barely two years in the Senate compared to Kennedy's seven when he sought the White House, Obama bathes in his reputation as an electrifying speaker at the last Democratic convention and on the campaign stump last fall. His unusual background as an African-American of mixed racial heritage has further shone a bright light on him.

While a fellow named Dan Quayle proved that comparisons with John Kennedy can be risky politics, Obama's youthful leap into presidential politics in some respects parallels Kennedy's 1960 plunge.

Like Kennedy, who faced competition from the dominant Democrats in the Senate of the day, Obama will probably encounter today's most celebrated party colleague in the former first lady, poised to seek a return to the White House.

Also like Kennedy, who despite his eight years in the Senate and six in the House was regarded as a greenhorn in foreign affairs in 1960, compared to globe-trotting Vice President Richard Nixon, Obama has no real foreign-policy experience to offer, at a time America's reputation in the world is on the rocks.

But seldom since Kennedy has a presidential hopeful burst on the scene with the kind of tailwind that already is blowing behind the Illinois freshman senator. He has been drawing record crowds in politically savvy states like New Hampshire usually blase about White House-seeking politicians.

Kennedy, known later for his charisma that eventually had voters waxing ecstatic about him, generated little such excitement in 1960 when he first walked the main streets of West Virginia seeking a toehold against the more seasoned Humphrey. It came only as he campaigned for months with a youthful vigor that captured public fancy.

The major hurdle Kennedy had to clear was concern about electing a Roman Catholic to the presidency, which was considerable then but is irrelevant today. For all the progress against racial discrimination today, Obama's candidacy will be a fresh test of that progress, and perhaps a much more difficult one than Kennedy's in 1960.

Obama, asked last October whether he was ready to be president, took note of the "long and rigorous" primary election process and said he knew he'd be "run through the paces pretty good." By opening the door to a candidacy now, he is assured of it. In addition to having to face press and public scrutiny all this year, once 2008 starts he will encounter more than 20 primaries in the first weeks alone.

Kennedy by contrast faced no such marathon gamut. He didn't announce his candidacy until the second day of January in 1960, and had seriously contested primaries against Humphrey in only two, in Wisconsin and West Virginia. Johnson meanwhile entered no primaries.

But Obama as an unknown quantity now, even more than Kennedy was then, may need the abundant time to reassure voters on their doubts and reservations about himself, as Kennedy managed to do then.

Jules Witcover's memoir, "The Making of an Ink-Stained Wretch," has just been published by Johns Hopkins University Press.