Hillary Rodham Clinton, Barack Obama and John McCain have constructed personas that reflect their biographies and political agendas. They can be revealing, but remain incomplete.

Senator John McCain, right, had conversations about being Senator John Kerry’s running mate in 2004.

By ELISABETH BUMILLER

WASHINGTON - Senator John McCain never fails to call himself a conservative Republican as he campaigns as his party’s presumptive presidential nominee.
He often adds that he was a “foot soldier in the Reagan revolution and that he believes in the bedrock conservative principles of small government, low taxes and the rights of the unborn.
What Mr. McCain almost never mentions are two extraordinary moents in his political past that are at odds with the candidate of the present: His discussions in 2001 with Democrats about leaving the Republican Party, and his conversations in 2004 with Senator John Kerry about becoming Mr. Kerry’s running mate on the Democratic presidential ticket.
There are divergent versions of both episodes, depending on whether Democrats or Mr. McCain and his advisers are telling the story.
The episodes reveal a bitter period in Mr. McCain’s life after the 2000 presidential election, when he was, at least in policy terms, drifting away from his own party.
They also offer a glimpse into his psychological makeup and the difficulties in putting a label on his political ideology .
“There were times when he rose to the occasion and showed himself to be a real pragmatist, said Tom Daschle, the former Senate Democratic leader who met with Mr.
Mc- Cain in 2001 about switching parties and who is supporting Senator Barack Obama.
“There were other times when he was motivated by political goals and agendas that led him to be much more of a political ideologue.
Such swings are common in politics, but for Mr. McCain, Mr. Daschle said, “those swings have been far more pronounced and far more frequent.
In the spring of 2001, Mr. McCain was by most accounts still angry about the smear campaign that had been run against him when he was campaigning for the Republican presidential nomination in the South Carolina primary the previous year.
He had long blamed the Bush campaign for spreading rumors in the state that he had fathered a black child out of wedlock, which Bush aides denied.
Mr. McCain was also upset that the new White House had shut the door on hiring so many of his aides.
Mr. McCain had begun to ally himself with the Democrats on a number of issues, and had told Mr. Daschle that he planned to vote against the Bush tax cuts, a centerpiece of the new president’s domestic agenda.
Still, Democrats were stunned one Saturday in late March when, by their account, John Weaver, Mr. McCain’s longtime political strategist, reached out to Thomas J.
Downey, a former Democratic congressman from Long Island in New York, who had become a lobbyist with powerful connections on Capitol Hill.
In Mr. Downey’s telling, Mr. Weaver posed a question to him over lunch that left him stunned.
“He says, ‘John McCain is wondering why nobody’s ever approached him about switching parties, or becoming an independent and allying himself with the Democrats, Mr. Downey said in a recent interview.
“My reaction was, ‘When I leave this lunch, your boss will be called by anybody you want him to be called by in the United States Senate.’ Mr. Weaver recalls the conversation differently.
He said that Mr. Downey had told him that Democrats, eager to find a Republican who would switch sides and give them control of the evenly divided Senate, had approached some Republican senators about making the jump.
“I stated they couldn’t be so desperate as they hadn’t reached out to McCain, Mr. Weaver said in an e-mail message recently.
Whatever transpired, Mr. Downey raced home and immediately called Mr. Daschle.
It was the first step in what became weeks of conversations that April between Mr. McCain and the leading Democrats, among them Senator Edward M. Kennedy of Massachusetts and John Edwards, then a senator from North Carolina, about the possibility of Mr. Mc- Cain’s leaving his party.
Mark Salter, one of Mr. McCain’s closest advisers, said that Mr. McCain, although flattered, never took the idea of leaving the party seriously.
But less than three years later, Mr. McCain was once again in talks with the Democrats, this time over whether he would be Mr. Kerry’s running mate.
In an interview with a blog last year, Mr. Kerry said that the initial idea had come from Mr. McCain’s side, as had happened in 2001.
Two former Kerry strategists said recently that Mr. Weaver went to Mr. Kerry’s house in Georgetown, outside Washington, D.
C., a short time after Mr. Kerry won the Democratic nomination in March and asked that Mr. Kerry consider Mr. McCain as his running mate.
(Mr. Weaver said in his e-mail message that the idea had come from Mr. Kerry.) Mr. McCain said in February that he had dismissed the vice-presidential offer out of hand.
“He is, as he describes himself, a liberal Democrat, Mr. McCain said of Mr. Kerry when he was asked about the episode by a participant at a public forum in Atlanta.
“I am a conservative Republican.
So when I was approached, when we had that conversation back in 2004, that’s why I never even considered such a thing.
Three former Kerry strategists said that Mr. McCain had not immediately dismissed the notion of sharing the Democratic ticket.
“McCain did not flat-out say no, regardless of what he’s saying now, said one strategist who asked not to be named.
“He was interested in this discussion.