Tomorrow night, President Bush and Senator Kerry will square off for their third and final debate in Arizona. The Grand Canyon State used to be bedrock Republican, but demographic shifts have transformed the state’s political landscape in ways that serve as a warning to both parties as they look to the future.

Independence was the watchword of the American frontier, and this spirit has been resurgent in voting patterns throughout Arizona. In John McCain’s home state, voters are now registering independent at a greater rate than any other political affiliation and the gap between Democrats and Republicans is closing. The result is a state that is less politically predictable than in years past, growing fast, and increasingly open to electing centrist Democrats or Republicans.

Between 1948, when Arizona pulled the lever for Harry Truman, and when the state’s voters cast their lot with President Clinton in 1996, Arizona was the only state to vote for a Republican presidential candidate every election. But a lot changed in the intervening decades. The state has grown to more than 5 million from 700,000. Maricopa County, where Phoenix is located, used to be a fairly desolate agricultural center. Now it is one of the fastest growing counties in the nation, and Phoenix recently eclipsed Philadelphia to become the fifth largest city in America. Two out of three Arizonans were born in another state.

With these seismic population shifts have also come political shifts. The conservative icon and 1964 presidential candidate Barry Goldwater hailed from the state, but over his 30-year Senate career he became more identified with the libertarian edge of the movement – supporting tax cuts and gay rights – than with the religious right. A longtime Arizona congressman, Morris Udall, was a Democrat who came in second to Jimmy Carter in the 1976 presidential primary, but he was known for his principled nonpartisanship as well as his defense of the environment and pioneering support for campaign finance legislation. In more recent years, Senator McCain, a Republican, has become a national symbol for political independence.

The independent streak in Arizona’s leaders is a reflection of the electorate. Over the past decade, they’ve adopted an independent redistricting commission to reduce partisan gerrymandering and opened the primary process to allow independent voters to participate. As a result, the number of independent voters in the state has increased to 23% today from 12.5% a decade ago.

This trend has grown even more pronounced in the partisan heat of a polarized election year. According to research done by Arizona Republic reporter Elvia Diaz, since January of 2004 nearly 70,000 new voters registered as Independent in Maricopa County, as opposed to 60,000 Republicans, 52,000 Democrats, and 1,600 Libertarians. The Economist memorably characterized this overall shift in the electorate by writing, “In Goldwater’s day, the typical new Arizonan was a wealthy Republican voter from the mid-west who had come to retire in the air-conditioned sun and cared about defense and taxes. Now it is a 30-something Democratic-leaner from California, who has come to work and cares about schools, crime, the cost of living and the environment. These are classic swing voters – fiscally conservative, socially liberal.”

How is this shift reflected in the policies embraced by the Arizona electorate? Voters in this heavily Hispanic state overwhelming supported a proposition to end bilingual education programs and replace them with a one year English immersion class. Arizona has the highest percentage of charter schools in the nation, with 20% of the total. Overall, Arizona cut state taxes sharply over the past decade, but approved targeted increases to fund education, tourism, and a sports stadium. When Rep. Jim Kolbe became the first openly gay Republican in Congress it caused barely a ripple in his district – he won his next election by nearly 70%. But when a ballot referendum passed that was seen as anti-immigrant, it caused a backlash against some Republicans that led to the 2002 election of a self-styled “conservative Democrat,” Janet Napolitano, as governor in 2002. Ms. Napolitano, who is originally from New York, had been Arizona’s state attorney general.

After Mr. Clinton’s 1996 victory, Mr. Bush came back to win the state by six percentage points in 2000 over Vice President Gore, who won neighboring New Mexico by just over 300 votes. Polls show Mr. Bush pulling ahead in Arizona, but a competitive race in the first congressional district – covering more land than the state of Illinois and dominated by a Navajo reservation – could make things closer come Election Day.

On Wednesday night, the candidates will be addressing an audience whose state reflects the shifting fault lines in American politics. To win the election and to govern effectively, they will need to address their concerns and reach out to Americans who lie beyond either party’s established base.