Big national holidays are about celebrating common ideals and aspirations; national politics often turn on how those ideals and aspirations match up against the realities of daily life.

So it may come as no surprise that the Fourth of July in an election year can be a time of dissonance, when pride and trepidation march side by side down Main Street.

Consider these snapshots from Independence Day 2012, all from states that are going to be crucial to outcomes this fall:

At a parade in Celebration, Fla., the Disney-built community southwest of Orlando, small-business owner Greg Iwanski said that he respects the Supreme Court’s recent opinion that the Affordable Care Act is constitutional — but that it only reinforces his desire to elect Republican Mitt Romney to get rid of the unpopular health-care law.

“This whole entitlement era has got to stop, or we’ll become Greece,” said Iwanski, who was wearing a star-spangled red-white-and-blue shirt. “It’s that serious.”

At the Union House bar in Parma, Ohio — famous for its pirogi — owner Doug Henderson insisted that Greece offers a reason to reelect President Obama.

“If General Motors and Chrysler Corporation had gone under the way [Romney] wanted them to, I think it would be in the tank,” he said of the U.S. economy. “I think we’d be Greece, Spain and Ireland right now.”

On the flag-lined central thoroughfare of downtown Leesburg, Va., law student Danielle Fein got a chance on Tuesday to express her fears to her former governor, who wants to be her future senator.

“We keep hearing it,” she told George Allen, the Republican Senate nominee. “ ‘Be prepared for an 18-month job search.’ ”

And on the outskirts of Las Vegas, Dennis Villanueva, who voted for Obama in 2008, was contemplating not the November election but his upcoming move out of state. Having seen his income in a casino bar drop by two-thirds, to $20,000 a year, he has had to walk away from two houses.

“I just gave up,” he said.

Anger, relief, anxiety, despair — this is what America sounded like on the 236th anniversary of the declaration of the shared purpose that inspired 13 colonies to break away from a domineering king across the ocean.

The intervening years, as Obama pointed out Wednesday at a naturalization ceremony for active-duty military, produced a Constitution, a Bill of Rights, a civil war, and ultimately the extension of voting rights to women and African Americans.

“Even now, we’re still perfecting our union, still extending the promise of America,” the president said.

But Independence Day found the nation more divided politically than at any time in memory.

Between red states and blue states, the debate rages: Has the government grown so big that it smothers us, or is it too small a raft to keep the most vulnerable from drowning?

Should we be more worried about the care and security promised our parents, or the debt we are leaving our children?

And in a nation built by wave after wave of mostly European immigrants, who should get to be an American? That issue has returned to the fore at a moment when the country has rounded a demographic corner: Whites no longer account for a majority of births.

This year’s election could very well turn on those questions — though it is not likely to settle them.

And geography will be at least as important as ideology.

Most of the electoral battle will be fought in about a dozen swing states. Four of them — Florida, Ohio, Virginia and Nevada — also are the settings of intense Senate races. They will help determine control of that chamber and, with it, how much leverage the next president will have.

Obama won these four states in 2008. But they now have Republican governors, all elected in the past four years, showing how unsettled their electoral landscapes are. They are being bombarded with political advertising, much of it negative, from the presidential campaigns and from outside groups.

Here is what their emerging political contours look like.

Nevada: Economics and demographics

On the southwestern edge of Las Vegas, Dave Graziano contemplated the abandoned tan stucco house next door to the one where he is renting a room. It was decorated for the season not with bunting and flags, but with a flier advertising the services of a real estate agent specializing in short sales and a postcard from a conservative Republican candidate who promised to oppose “job-killing tax hikes.”

“Obama, instead of giving money to the car companies, he could have saved a lot more jobs” by channeling resources to the devastated local construction sector, Graziano said.

Originally a backer of former House speaker Newt Gingrich, he is now supporting Romney and said he will also vote for Republican Sen. Dean Heller over the Democratic challenger, Rep. Shelley Berkley.

Nevada leads the nation in unemployment, though it showed some improvement in May, and has been one of the states hit hardest by housing foreclosures.

It’s not a good environment for incumbents. But Orlando Fernandez, 24, laid off two weeks ago from his job at a vitamin warehouse, still has faith in Obama. “The country needs a lot of time to get improvements. It’s not a one-, two-, three-year thing,” he said. He plans to vote Democratic down the rest of the ticket, too.

The state is also seeing a demographic shift: Hispanics made up 3 percent of Nevada voters in 1996, but they accounted for 15 percent in 2008.

As members of Rancho High School’s Hispanic Student Union sold fireworks from a 7-Eleven parking lot in Las Vegas to raise scholarship money, they also handed out fliers about Obama’s decision to stop deporting young people brought to the United States illegally as children.

“We’re all Americans, despite what some people would have us believe,” said Isaac Barrón, the student group’s adviser.

Another key electoral demographic in Nevada has reason to be excited about the election.

At the North Las Vegas stake of the Church of Jesus Christ of Latter-day Saints, the nation’s independence was celebrated with a traditional July 3 evening of patriotic songs.

Mormons make up about 7 percent of Nevada’s population but vote in large numbers. Polls showed that about a quarter of the participants in Nevada’s February GOP caucuses were Mormons, and Romney won them handily. He also won the caucuses.

Drew Leavitt, a fourth-generation Las Vegas native, said his son will turn 18 before November and is excited to cast his first-ever vote for a fellow Mormon. “He stands for the same beliefs,” Leavitt said.

But he and several other Romney backers said they will also support Democrat Berkley, a longtime Las Vegas congresswoman, in the Senate race.

“She’s done a fantastic job representing us. Regardless of political party, she’ll have my vote,” Leavitt said.

Florida: A referendum on health care

If there is a ground zero of resistance to Obama’s signature legislative accomplishment, it is Florida, where Gov. Rick Scott (R) announced this week that he will turn down federal money for the expansion of Medicaid, undercutting the health-care law’s chief means of covering poor people who currently lack coverage.

“I’m focused on getting them jobs,” Scott said of Floridians. “That’s how you get health care.”

But that stance probably won’t sit well with voters like Edwin Sotomayor, 51, of Orlando. He is disabled and paying for his health care out of pocket while he waits to get on Medicaid. “Right now,” Sotomayor said, “I just want any kind of coverage.”

Embattled Democratic incumbent Sen. Bill Nelson predicted that the governor will back down once it becomes clear to Floridians that he is rejecting a good deal from the federal government.

“He’ll change his mind once he realizes what the law is,” Nelson said. “The federal government share of that is 100 percent [for the first few years, and 90 percent after that], and that means a lot of health care for disabled and poor people.”

But many in Florida are uneasy about the health-care law, especially the penalty it imposes on those who choose to go without medical coverage. In the 24 hours after the Supreme Court declared the law constitutional, the Romney campaign raised $350,000 from 4,000 donors in Florida, 2,500 of whom had never given to the presumed Republican nominee before, a campaign official said.

“I’m happy with the overall concept, but I don’t agree with the individual mandate,” said Jamie Mumblow, 33, of Orlando, a student and mother of two who works as a legal assistant. “Most people can’t afford it, and you’re making them buy something they can’t afford.”

In less than two months, Florida — and more specifically, the up-for-grabs area surrounding Tampa — will take center stage in the political world when the GOP holds its national convention there.

Republicans hope the attention will give Romney, as well as their Senate nominee, Rep. Connie Mack, a crucial boost in that part of the state. It sits at the western end of the Interstate 4 corridor, which is famous for its mother lode of swing voters. Obama won narrowly in the I-4 corridor in 2008, after George W. Bush carried it in 2000 and 2004.

Ohio: Good economic news and campaign money

For a guy who is trailing by double digits in the polls, Josh Mandel speaks with great confidence about his ability to bring down a sitting senator.

“One of the reasons we’re going to beat Sherrod Brown is because we’re earning the support of blue-collar Democrats throughout the state in places like Parma,” Mandel, a Republican, said as he sipped a big glass of cold water at a strip-mall diner there. “They may have a D next to their name, but so many people go hunting on Saturday, church on Sunday, believe in fiscal responsibility and peace through strength.”

The 34-year-old Ohio state treasurer has also been the beneficiary of a torrent of advertising against the Democratic incumbent. As of June, outside groups were spending four times as much against Brown — who has one of the most liberal voting records in the Senate — as they were against Mandel.

Mandel has also raised more money than any other GOP Senate contender this year except for Scott Brown in Massachusetts.

But Ohio’s Senate race, like others elsewhere, is also likely to turn on the larger dynamic of the presidential contest. The Obama campaign has more offices in Ohio — close to 40 — than it does in any other state.

Ohio voters express annoyance with the attack ads from both sides, though often the facts they cite in response to questions about the race come directly from those ads.

The latest spot from the Obama campaign centers on Romney’s record, as a founder of the private-equity firm Bain Capital, of investing in companies that outsourced jobs overseas. And the president will arrive in the state Thursday to kick off what the campaign is billing as a “Betting on America” bus tour.

What could help his prospects more than anything else, however, is the fact that Ohio’s economy is improving. The same is true in other swing states, although Republicans are eager to give the credit to their GOP governors.

“It’s positive feeling out there. You can see it in everything from the restaurants that are open to the activity,” said Parma’s clerk of courts, Martin Vittardi, a Democrat and an Obama supporter. “Go to downtown Cleveland. It’s thriving.”

Virginia: High stakes and familiar faces

That Virginia is now a swing state is a remarkable development in itself.

In 2008, Obama was the first Democrat to win it since Lyndon B. Johnson 44 years before. In gubernatorial and midterm congressional elections since then, it has swung sharply back to the Republican column.

Within the state, the areas that matter most are the ones that are growing fastest.

“Look to the outer suburbs in Northern Virginia, particularly Loudoun and Prince William,” said Mark J. Rozell, a professor of public policy at George Mason University. “These have been swing counties in recent election cycles. As elsewhere, the economy and jobs will be the leading issues for most voters in these states.”

The presidential race has overshadowed a Senate contest that pits two former governors, Republican George Allen and Democrat Timothy M. Kaine, against each other. The familiarity Virginians feel toward both has taken some uncertainty out of the equation; people know how they feel about their options. In a May poll by The Washington Post, Kaine and Allen were tied.

“That’s a pretty good choice,” said Bob Parker, 66, an Arlington County retiree who was climbing out of his car to go to the city of Fairfax’s Fourth of July parade. “Neither of them is ‘out there,’ and they’ve shown they can govern.” He said he is leaning toward Allen.

At Dale City’s parade, Kaine was sweating through his shirt as he moved from one side of the street to the other, trying to catch every hand he could.

“Even the folks on the other side are polite,” he said of the reaction he receives in this annual ritual.

As Allen shook voters’ hands in Leesburg, he promised he would undo what Obama has done, repealing the health-care law and opening up drilling off Virginia.

Kaine, meanwhile, was pushing a message of bipartisanship.

“That’s what they want to hear over and over,” he said of the voters he meets. “We need more people who want to work together.”

But in a country that is deeply divided along party lines, perhaps the rarest voter of all these days is one who is willing to offer the other side the benefit of the doubt.

“I don’t think the economy is improving as much as it could have,” said Roger Caulkins, 81, whose brother owns a Leesburg jewelry store that Allen visited Tuesday.

Caulkins said he will probably vote Republican in November, but he added: “It’s hard to just blame one person — like the president.”

This story was reported by staff writers Jeremy Borden in Virginia, T.W. Farnam in Florida, Michael Laris in Nevada and Rachel Weiner in Ohio. Tumulty reported from Washington.