By Andy Sullivan

WASHINGTON (Reuters) - This year's presidential election could be among the closest in U.S. history, as President Barack Obama and Republican Mitt Romney are locked in a virtual dead heat. For a growing number of voters, however, it will not be much of a contest at all.

Roughly 22 percent of the U.S. population lives in the nine politically divided states that are likely to determine the outcome of Tuesday's election - and that have been the focus of the candidates' campaign stops and advertising.

That has left four-fifths of Americans effectively on the sidelines, the highest proportion in more than century.

It's a reflection of how Americans elect their president not by popular vote, but through a state-by-state process known as the Electoral College. The winner of each state typically is awarded that state's electoral votes, with 270 electoral votes needed to clinch the White House.

Over the past four decades, presidential elections have played out on a shrinking battlefield, as political parties have become more ideologically unified and Americans increasingly have moved to communities where their neighbors share their political views, analysts say.

As a result, most states can be taken off the table - as either particularly friendly to a candidate, or not - before a presidential campaign even begins.

With the South and the Plains states firmly in the Republican column and the Northeast and West Coast solidly Democratic, elections since 2000 have come down to a few states that, for one reason or another, remain politically diverse and up for grabs.

This year, Obama and Romney focused the bulk of their television advertising and ground-level campaigning on nine states: Nevada (6 electoral votes), Colorado (9), Iowa (6), Wisconsin (10), Ohio (18), New Hampshire (4), Virginia (13), North Carolina (15) and Florida (29). Romney has made a late bid for Pennsylvania and its 20 electoral votes, but polls there have shown Obama with a consistent lead.

Some of the most competitive states, such as Virginia and Colorado, have become that way as more Democratic-leaning Hispanics and college graduates have moved in. Others such as Ohio and Iowa have been relatively untouched by demographic shifts, and the two parties have remained at a rough parity in those states.

And some states have been split down the middle.

In Wisconsin, the state capital of Madison is one of the most Democratic cities in the country, while the suburbs of Milwaukee are now among the most Republican.

Competitive presidential races in the 1960s and 1970s involved states that encompassed more than half the U.S. population. Since 2000, the presidential race has been competitive in states where roughly one-quarter of U.S. voters have lived.

Political analysts say the narrowed geographic scope of the presidential campaign is not just a symptom of increased polarization - it is also a reason the gap keeps widening. The dynamic encourages presidential candidates to ignore wide swaths of the nation and can make it more difficult for the winner of the election to govern effectively.

"Politics now isn't about issues, it's really more of a tribal affiliation," said Bill Bishop, who examined the increasing geographical polarization of the country in his 2008 book, "The Big Sort."

AN IDEOLOGICAL SHIFT

Between the 1880s and 1960, close presidential elections were rare in the United States. Since the 1960s, Americans increasingly have moved into communities that mirror their own cultural preferences, Bishop found.

A computer programmer who wants to cycle to work and eat organic produce might move to San Francisco, while someone with the same skills who wants plenty of parking and a thriving evangelical community might choose a Dallas suburb, Bishop said.

Over the same time period, political parties have become more ideologically consistent.

While Republican voters now are much more likely than Democrats to oppose abortion, the two groups showed no difference on the issue in 1988, said Columbia University professor Andrew Gelman.

"The parties are definitely more ideological than they used to be, and the voters are, too," Gelman said.

This ideological and geographical sorting has been most pronounced at the local level, but it has played out at the state level as well, as southern conservatives have switched to the Republican Party and northeastern moderates have been pushed to the Democratic Party as Republicans have emphasized conservative stands on social issues.

As politically divided as the nation is overall, presidential races are now much less likely to be competitive at the state level.

The average state margin between the winning and losing presidential candidate in close elections widened steadily from 8 percentage points in 1960 to 14.8 percentage points in 2004, according to research by Alan Abramowitz of Emory University and Kyle Saunders of Colorado State University.

The presidential battleground as a whole has shifted during this period. Virginia, for example, had been a reliably Republican state in presidential races from the 1950s until Obama carried it in 2008.

At the same time, elections have steadily shrunk in scope.

Republican Richard Nixon campaigned in all 50 states in the 1960 campaign for the White House. He lost narrowly to Democrat John F. Kennedy, but the race was competitive in 20 states, where the margin of victory for either candidate was narrower than 5 percentage points for either candidate. Continued...