Over the next 65 days, Americans will be presented with two very different visions of the role of government, one anchored in the verities of the past, the other urging something that hasn’t been tried for a while.

The first will be articulated by Sen. John McCain of Arizona, an old warrior who embodies duty, honor and country. At his side will Sarah Palin, moose-hunting mom and daughter of the Alaskan frontier.

Over the next four days at their convention in St. Paul, Minn., Republicans will nominate Mr. McCain and Ms. Palin as president and vice president. They’ll offer a kind of post-feminist John Wayne vision of America as a place where rugged individuals will continue to unleash the forces of creative capitalism as they’ve always done.

The second vision will be articulated by Sen. Barack Obama of Illinois, the personification of a new polyglot, transracial America, and Sen. Joe Biden of Delaware, who’s been agitating for change for most of his 65 years.

Last week in Denver, the Democrats nominated the Obama-Biden ticket as ready, indeed eager, to remake America — in Mr. Obama’s words — as “one American family,” where the highest goal is the common good.

The race is on.

The messages are shorthand, and the whole truth is far more complicated and nuanced than the message makers would have us believe. America is 300 million people with almost that many sets of interests. The electorate will be sliced, diced and microtargeted in a million ways between now and Nov. 4, the messages amplified and distorted by hundreds of millions of dollars worth of television commercials. Much of the message won’t stand up to scrutiny.

Consider Mr. McCain’s choice of Ms. Palin as his running mate, Calamity Jane to his Wild Bill Hickok. Here’s the standard bearer of the party whose base is old white men choosing a young woman as his running mate, a bold attempt to capitalize on cracks inside that happy Democratic family. Sen. Hillary Clinton of New York, in perhaps the best speech of her political career, did her best last week to spackle over those cracks, but it remains to be seen whether the patches will hold.

It is insulting to think that Mrs. Clinton’s only attraction to voters was her gender. And aside from their paired X chromosomes, Mrs. Clinton and Ms. Palin appear to have little in common. The former has been in politics most of her life, the latter only since 1992, when she joined the Wasilla, Alaska, City Council the same year Mrs. Clinton was helping her husband become president of the United States.

One is pro-choice; the other pro-life. One is anathema to family-values voters; the other is a champion. One was a feminist lawyer; the other was elected Miss Congeniality in the 1984 Miss Alaska contest.

Ms. Palin’s elective experience is limited to town politics in Wasilla, an Anchorage suburb with a population of 8,471, and 20 months as governor of a state with two-thirds the population of St. Louis County. So in his first presidential-level decision, John McCain placed her in position to be within one beat of a 72-year-old heart from the presidency.

And to think, the motto of the Republican Convention is “Country First.”

Mr. McCain may have done the Democrats a favor in selecting Ms. Palin. By comparison, with eight years in the Illinois Senate and four in the United States Senate, Mr. Obama now looks like a grizzled veteran. His choice of Mr. Biden, a 36-year Senate veteran, looks … dare we say it … presidential.

Thanks to Mrs. Clinton’s graciousness, however belated, the Democrats managed to get out of Denver looking unified. Their convention was carefully choreographed to allow all the fractious elements of the party a moment in the sun, though plenty of sunscreen was applied. When Rep. Jose Serrano from the Bronx submitted the text for the speech he delivered at Monday’s session for approval, the Obama campaign scrubbed it of the line, “let us build bridges of friendship and cooperation with our Southern neighbors.”

No sense raising the sticky issue of immigration reform, particularly as Mr. McCain himself supports (or at least he used to) giving undocumented immigrants a pathway to U.S. citizenship.

A bigger problem for the Democrats this fall is the very audaciousness of what Mr. Obama represents. Not only is he the first African-American to be nominated for president by a major party, but also he is challenging Americans to confront change and embrace it.

His speech Thursday night was not soaring rhetoric but a litany of the dislocation that Americans feel: “[O]ur nation is at war, our economy is in turmoil and the American promise has been threatened once more. Tonight, more Americans are out of work and more are working harder for less. More of you have lost your homes and even more are watching your home values plummet. More of you have cars you can’t afford to drive, credit card bills you can’t afford to pay and tuition that’s beyond your reach.”

Mr. Obama dealt forthrightly with the fact that fundamental assumptions of the last 50 years have been shaken: Gasoline no longer is cheap and plentiful, good jobs are disappearing and higher education may be out of reach for many families. Economic models aren’t holding, social boundaries are falling and African-Americans, Hispanic Americans and Asian-Americans are crowding the table, which feels threatening to those whose own place at the table no longer feels secure.

His message was that America has to change and adapt and build a bigger table to contain what he called “the promise of America.”

Mr. Obama and Mr. Biden will spend the fall laying the blame for America’s failure to adapt on President George W. Bush, and by extension, on Mr. McCain. They will try to sell the notion, embodied in Mr. Obama himself, that Americans of every race share common goals and aspirations and that government has a role in helping us achieve those goals.

“Our government should work for us, not against us,” he said. “It should help us, not hurt us. It should ensure opportunity not just for those with the most money and influence, but for every American who is willing to work.

“That’s the promise of America, the idea that we are responsible for ourselves, but that we also rise or fall as one nation; the fundamental belief that I am my brother’s keeper; I am my sister’s keeper.”

That’s not a new vision for America, but it is one that has fallen on hard times in the last 40 years. The “Me Generation” did damage to it in the 1960s and 1970s; the Reagan revolution undermined it in the 1980s; the booming economic times in the 1990s made it less pressing; and the Bush administration of this decade all but killed it.

Mr. McCain and Ms. Palin have the easier job, selling the idea that we can have change without changing very much, that the market will cure our ills with just a tweak here and there, that if government would just stay out the way, the good ship America will right itself.

These two threads — the common good and individual rights — have run through American history since the Pilgrims hit our shores. Balancing them always has been the trick. It remains so this fall.