But if Mr. Obama becomes the Democratic presidential nominee, he is sure to face an onslaught from Republicans and their allies that will be very different in tone and intensity from what he has faced so far.
In the last few days alone, Senator John McCain has mocked a statement Mr. Obama made about Al Qaeda in Iraq. The Tennessee Republican Party, identifying him with his middle name as Barack Hussein Obama, suggested that his foreign policy would be shaped by people who are anti-Semitic and anti-Israel.
The Republican National Committee issued a statement on Wednesday invoking a questionnaire Mr. Obama filled out when running for Senate in 2004 to show that he once opposed cracking down on businesses that hire illegal immigrants.
Without using Mr. Obama’s name, President Bush, at a White House news conference on Thursday, assailed his willingness to meet Cuba’s new leader, Raúl Castro, without preconditions, saying that to do so would grant “great status to those who have suppressed human rights and human dignity.”
For much of this year, Mr. Obama has been handled with relative care by Mrs. Clinton and, before they dropped out, the other Democratic candidates. They generally do not have huge policy differences with him, and they have been wary of making a particularly harsh attack that winds up in a Republican television advertisement this fall.
Yet the shifting tone offers a glimpse of the Republican playbook as the party adapts to the prospect that it will be running against Mr. Obama rather than Mrs. Clinton.
It is a reminder that should Mr. Obama win the nomination, he will be playing on a more treacherous political battleground as his opponents — scouring through his record of votes and statements and his experiences before he entered public life — look for ways to portray him as out of step with the nation’s values, challenge his appeal to independent voters and emphasize his lack of experience in foreign policy and national security.
Some of this will almost certainly take the shape of the Internet rumors and whispering campaigns that have popped up against Mr. Obama since he got into the race, like the false reports that he is Muslim. Others will no doubt come from the types of shadowy independent committees that have played a big role in campaigns in recent years.
But others will simply draw on Mr. Obama’s voting record and speeches, interviews and debate appearances. Mr. McCain’s aides said their first line of attack would be to portray him as a liberal, and they have already begun pointing to a rating in The National Journal, based on his votes, of Mr. Obama as the most liberal member of the Senate.
Though Mr. McCain has vowed repeatedly to wage a tough if respectful campaign — he chastised a conservative talk radio host this week for disparaging Mr. Obama and invoking his middle name — his aides have left no doubt that they will draw sharp distinctions with him on issues that Mrs. Clinton has never been able to use. Foremost among them is Iraq.
“Her fundamental problem is, in a Democratic primary, she can’t make an issue contrast against him,” said Steve Schmidt, a senior adviser to Mr. McCain. “On the Republican side, we’ll have a very significant issue contrast against him. When you look at issues — taxes, spending, judges, health care and national security — there is a divide as deep and wide on those issues as the Grand Canyon.”
Mr. Obama’s record is not as long as Mrs. Clinton’s, or as potentially rich, for an opponent looking for damaging votes or quotes. But there is still plenty to work with. Some cases are simple let’s-go-to-the-video moments, like Mr. Obama’s statements that he would support giving drivers’ licenses to illegal aliens or would support raising taxes to shore up Social Security, lines of attacks that Republicans are already employing.
Others — like a suggestion that Mr. Obama opposed the USA Patriot Act or supported a ban on handguns — might be subject to dispute by Mr. Obama, who would argue they were yanked out of context or did not take into account the subtleties of shaping legislation. (Nuance is usually a weak defense in political campaigns.)
Should Mr. Obama win the nomination, his candidacy could well be a test of whether these tactics still work or whether, used against a candidate who is trying to cultivate an appeal that transcends policy specifics, would fall flat this time. The fact that Mr. McCain felt compelled to rebuke some critics of Mr. Obama over the past few days suggests he might see a danger in attacking too aggressively.
But Mr. McCain clearly will not control all of the voices that could oppose Mr. Obama, from bloggers and talk radio hosts to other elected officials. Even parts of the Republican Party apparatus can transmit messages that the presidential nominee cannot or will not.
After the Republican National Committee rebuked the Tennessee Republican Party for a news release this week using Mr. Obama’s middle name and a picture showing him in a traditional African outfit — Mr. McCain also expressed his disapproval — the state party removed the middle name and the picture.
But for at least some period of time, it left the text of the release on its Web site, seeking to link Mr. Obama to the views of some of his most controversial supporters, including Louis Farrakhan of the Nation of Islam.
“They will try to rerun old races and battles and divide along traditional lines,” Mr. Axelrod said. “I think the country is eager for something else. And I think the country is not going to be so easily distracted. We are prepared to deal with whatever they offer.”
“I understand very, very well how facts can be manipulated,” he said. “I’m not going to get into specifics, but I know his record well, we know his record well, and we understand the areas that they might try to exploit. But I also am very, very confident that we can parry those kind of tactics effectively and show the same appeal with independent voters and some Republican voters that he has in Illinois.”
Mrs. Clinton has been arguing for months that she would be the stronger opponent against the Republicans than Mr. Obama because her record is already well known and his is not. This is part of the case Mrs. Clinton has been making to Democratic superdelegates in the final stand of the campaign.
“He regularly goes out there and says he’s the person who can beat John McCain,” said Mark Penn, Mrs. Clinton’s chief strategist. “But the truth is, if he is ever in a general election, a lot of positions he took in 2003 and 2004 will come back to haunt him in a big way and a lot of the vetting that didn’t happen will happen. The independent and Republican support that he has had will evaporate really quickly.”