HENDERSON, Nev. » The presidential campaigns and their allies are focusing mainly on nine swing states, bombarding them with commercials in the earliest and heaviest concentration yet of advertising in modern politics.

With so many resources focused on persuading an ever-shrinking pool of swing voters like those here in Nevada, the 2012 election is likely to go down in history as the one in which the most money was spent reaching the fewest people.

Much of the heaviest spending has been not in big cities with large and expensive media markets but in small and medium-sized metropolitan areas in states with little individual weight in the Electoral College: Cedar Rapids and Des Moines in Iowa (six votes); Colorado Springs and Grand Junction in Colorado (nine votes); Norfolk and Richmond in Virginia (13 votes). Since the beginning of April, four-fifths of the commercials run about the presidential campaign have been in television markets of modest size.

Nowhere is this more apparent than in southern Nevada, where a costly and contentious fight is playing out for just six electoral votes.

Already, ads about President Barack Obama or Mitt Romney have been broadcast nearly 6,000 times in and around Las Vegas since April 11, the most of any media market in the country during that period, according to the Kantar Media’s Campaign Media Analysis Group. And the $5 million spent by both sides during that eight-week stretch translates into the highest rate of spending per electoral vote anywhere by far. Underscoring the state’s importance this year, Obama campaigned in Las Vegas on Thursday; Romney visited last week for a rally and a fundraiser.

All this effort is to reach just 1.4 million registered voters, a sign of how tight this election is expected to be. And it is a sign of how the country’s growing partisan divide has redrawn the political geography, with fewer states than ever not firmly designated "red" or "blue."

A study released this week by the Pew Research Center found that the differences between Republicans and Democrats on a wide range of questions — like whether someone believes in God and what role the government should have in helping the poor — have never been starker in the 25 years since Pew began the survey.

These divisions play out on television sets across the sprawling desert developments here, where political ads start with the 4 a.m. newscasts. They appear alongside commercials for bankruptcy lawyers and the "sizzlin’ million-dollar giveaway" at a supermarket chain, reminders of how the economic recovery has skipped over much of Nevada.

In one campaign ad, a minute-long spot from the Karl Rove-backed group Crossroads GPS, a litany of Obama’s "broken promises" is punctuated by the sound of shattering glass. Another ad uses a ticking stopwatch to drive home the point that the national debt increases $1.4 million every second. It lays the blame squarely at the president’s feet ("borrowing from China for his spending," it says) despite sharp deficit growth under George W. Bush.

The Obama campaign’s presence is equally strong on the air, and it grew even heavier this week with a new $10 million ad campaign in Nevada and eight other states. The latest ad claims that as governor of Massachusetts, Romney had "one of the worst economic records in the country."

"It is unusual that so few states are in play from an advertising standpoint," said Will Feltus, senior vice president of National Media, which researches and plans advertising placement.

He said that the sheer amount of money in play this year is a factor. And because neither candidate is accepting public financing, which would put limits on how they spend, they can be more strategic about how they advertise.

"I think campaigns this year are marshaling their resources more carefully," he said.

Given the volatility of the electorate, the map could easily expand or contract in the weeks and months ahead. And advertising is only the most tangible sign of which states are competitive. There are other factors like voter registration and organization that play important roles.

But no recent general election advertising strategy has covered so little ground so early. In the spring of 2000, Bush and Al Gore fought an air war in close to 20 states. In early 2004, there were the "Swing Seventeen." And in 2008, the Obama campaign included 18 states in its June advertising offensive, its first of the general election.

In 2012, Nevada offers a clear test case for the economic debates and changing demographics that will determine this election. Nevada has the highest unemployment rate in the country, and one of every 300 homes here is in foreclosure — more than in any other state.

Hispanics, a group that voted heavily for Obama in 2008 and is aggressively courted by both sides this year, are a sizable constituency. Labor unions, another Democratic ally, are especially powerful here. Mormons, who make up an estimated 10 percent of Nevada’s electorate, are expected to turn out for Romney. And Nevada is also already a showcase for the influential role that outside political groups play in a post-Citizens United world.

Conservative political groups have spent heavily in recent weeks to counter the $20 million advertising blitz that the Obama campaign started last month in nine states. Groups like Crossroads GPS, the Koch brothers-affiliated Americans for Prosperity and the American Future Fund spent $20 million in May in 10 states — Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, North Carolina, Ohio, Pennsylvania and Virginia.

Since April, conservative groups have had a near-continuous presence on television in battleground states, providing Romney with cover while his campaign remained off the air. In most states the pattern was the same: One group’s ad buy picked up when another’s ended. In Colorado, for example, Americans for Prosperity started running commercials just as American Future Fund’s ads were tapering off. Then Crossroads stepped in after Americans for Prosperity’s buy wound down. The same has been true in Nevada, Virginia, Florida, Ohio and Iowa.

Meanwhile, the Romney campaign has advertised in only four states — Iowa, North Carolina, Ohio and Virginia — for a total of $4.6 million, according to an ad buyer who monitors spending. The Romney campaign has not said publicly where or how much it is advertising.

The fall promises to bring wall-to-wall advertising. Already, commercial time in October during NFL and World Series baseball games has been reserved by the campaigns and outside groups. And the Obama campaign is said to be inquiring about national television ads during the Summer Olympics, according to one person who monitors political ad buying.

By November, breaking through the clutter will be difficult. For now, at least, some campaigns seem to be see intent on pouring cash and time into Nevada.

But many voters seem to have already made up their minds.

"I have kids, and I don’t want to see their future compromised,"B said Joni Robison, a homemaker from Las Vegas,B as she stood waiting with her daughter and husband for a Romney rally to begin.

Four years ago, she was excited about Obama.

But, she said, "I’m disappointed in some of the promises he made that he wasn’t able to keep. I’m disappointed in his performance."