On a routine drive to the beauty salon, Robin Johnson had one of those life-happens moments: Her 13-year-old Durango, with 200,000 miles on the odometer, overheated and started sputtering. Convinced that the car was on its last legs, Robin and Scott Johnson scrutinized their already-tight family budget, looking for a way to fit in car payments.

The couple from Culpeper, Va., were already masters at scrimping.

At Christmastime, Robin unexpectedly needed prescription eyeglasses, so gifts came from a discount store. The couple spent the day before Thanksgiving price-checking turkeys at three stores.

But reliable transportation was not a luxury, so in late February the couple bought a used 2012 Dodge Caliber. As a result, they’ve fallen behind in the electric bills for March and April.

The Johnsons both work, earning $90,000 between them, not a princely sum but one that places the couple squarely in the middle of household incomes for the Washington region. But for the Johnsons and many other American families, being middle class means living paycheck to paycheck.

The couple’s retirement savings are meager. The college fund? Nonexistent.

The Johnsons, whose blended family includes three children under 18, are part of a drawn-out, disquieting shift that is recasting what it means to be middle class in America.

“I don’t know how the working class, anyone below the middle class, how they can survive,” says Scott, 48, who works as an IT engineer at a hospital. “Once some bills are paid and we’ve gone grocery shopping, we don’t have enough to pay the other bills.’’

The problems of an anxiety-ridden middle class have grabbed the attention of politicians and scholars — and become the centerpiece of President Obama’s second-term agenda. “Every single day, I’m going to fight for these priorities: To shift the odds back in favor of more working- and middle-class families and to keep America where you can always make it if you try,” he said in a recent address.

Wages for millions of American workers, particularly those without college degrees, have flat-lined. Census figures show the median household income in 2012 was no higher than it was 25 years ago. Men’s median wages were lower than in the early 1970s.

Meanwhile, many of the expenses associated with a middle-class life have increased beyond inflation. This includes college tuition, whose skyrocketing cost has laid siege to a bedrock principle of the American Dream: that your children will do better than you did.

A recent poll conducted by the Washington Post and the Miller Center at the University of Virginia found that 40 percent of those calling themselves middle class felt less financially secure than they were just a few years ago. Forty-five percent said they worry “a lot” about having enough money stashed away for retirement, and 57 percent said they worry about meeting their bills. Less than half said they expect their kids to do any better.

Fewer Americans find themselves in the heart of the middle class with every passing year.

In the mid-1970s, the majority of Americans were in the middle, with 52 percent earning the equivalent (in today’s dollars) of $35,000 to $100,000. Today, according to census figures, the share of households earning under $35,000 is virtually unchanged, 35 percent. The shift has occurred in the other two categories. Households with incomes over $100,000 have doubled, to 22 percent, while less than 44 percent are in the middle cluster.

The Johnsons’ $90,000 income is higher than the national household median of $51,000, as well as the $66,500 median in Virginia. But in the broader Washington region that Culpeper is part of, where the median income is $88,000, the Johnsons are just about average.

“On the one hand, $90,000 sounds like a lot to most middle-class Americans, because most Americans don’t earn that,” said Joseph Cohen, a sociologist at Queens College in New York City. “But the fact is, the median-income American does not do well in a lot of respects. One $5,000 home repair can wipe out their surplus for a year. A medical event, an auto repair or a temporary job loss can exert a large shock.

“America is a place where luxuries are cheap and necessities costly. A big-screen TV costs much less than it does in Europe, but health care will sink you.”

One factor behind the financial squeeze is that the middle class’s expectations — a house, music and dance lessons for the kids, the latest in home entertainment — have stayed the same or increased even as costs have soared. It’s particularly rough in the Washington area, where salaries are among the highest in the nation but so are housing and child care expenses.

“It’s a question of what your basic standards are,” said Cohen. “Basic standards mean living in a school district that’s half decent,” where houses are more expensive.

“One of four biweekly checks can go to child care, if it’s done illegally,” he added. “If it’s done legally, it’s much more.”

Perceptions of what’s middle class are more porous than the statistics suggest.

In the Washington Post-Miller Center poll, the figure people most commonly said it takes to be middle class is $75,000, meaning that for most Americans, being middle class is only an aspiration. Overall, people who said they consider themselves middle class were more likely to be white, suburban and earn more than the national median of $51,000.

Robin Johnson sets the bar higher.

“$150,000 a year,” she said, with no hesi­ta­tion. “If we had an extra $60,000 a year, we’d have some breathing room. I’d like to have some extra things. Not just look at them and drool.”

Scott and Robin Johnson were each divorced when they met through Girl Scouts. Scott’s 16-year-old daughter, Taylor, and Robin’s daughters, Emily and Charlie, who are 13 and 10, are all active in the organization. The parents became friends as they chatted online about scouting and fishing as well as the difficulties of starting anew while sharing custody with their exes.

Robin, 44, was raised in Texas and Ohio. Her father was a butcher. Her mother was a stay-at-home mom until the family split, and then she worked at a succession of blue-collar jobs. Robin joined the Army Reserves after high school and eventually earned a bachelor’s degree in accounting through online courses from Colorado Technical University.

Scott’s family moved so many times that he had attended 18 different schools around Northern Virginia by the time he was a high school senior. His father worked mostly as a small businessman who ran his own landscaping and taxi companies. After his parents divorced, Scott’s mother raised her three children on a $10,000-a-year job as a secretary with the federal government. She eventually rose to a top-level position in purchasing that paid her more than Scott earns now.

Like seven in 10 Americans, Scott does not have a college degree. “My goal is to do as well as my mother,” he said. “When you think where she came from and what she accomplished in her career, it’s amazing.”

When they started dating, Robin was burdened with $28,000 in student debt and strapped for cash.

Scott was doing better, though he was barely making ends meet. He, in effect, took a pay cut when he made a switch from hourly pay with overtime.

And he has bad credit, mostly the result of a dispute with a bank over a bill that he said he paid after 3 p.m. on the day it was due several years ago. He didn’t notice when the bank assessed a late-payment fee, which soon multiplied with interest and fees.

But he had enough money to come to Robin’s rescue when she called him after her car broke down while driving with her girls to Richmond. Scott picked her up and paid the repair bill.

They married last October, blending their two households in the four-bedroom house Scott had bought in 1997 in Culpeper, a town of 45,000 about 75 miles southeast of the District. Scott restructured his mortgage just in time to pay for a wedding photographer.

Living together has somewhat eased the Johnsons’ financial problems. Robin no longer had to pay $900 a month rent, which ate up the bulk of her salary from the Native American nonprofit organization where she answers queries from donors and members. Now her wages make up for what Scott lost when he stopped getting overtime.

“Together, we are better off than we were a year ago,” said Scott. “If we hadn’t moved in together, one of us would have lost the house.”

“Probably me,” said Robin, who was accustomed to spending as little as possible. When the time came to buy school photographs for her daughters, she usually ordered the smallest, cheapest package.

Scott, whose salary is more than triple Robin’s, insisted on ordering the biggest package.

“I don’t care if we don’t eat next week, I’m going to get the kids’ photos,” he said. “That’s what you hold onto.”

While they struggle to meet basic expenses, the Johnsons’ home is filled with the electronics that have become a standard part of middle-class life in the 21st century. For $90 a month, a satellite dish provides basic television service for their three flat-screen sets and for the WiFi connections Scott needs when he works from home. They have one laptop and three iPads, and each girl has a computer in her bedroom. The bill for four cellphones runs about $300 a month.

Like many American homeowners over the past several years, they got a break on the mortgage by refinancing to a lower interest rate, stretching the remaining payments from 14 years to 30. It cut the monthly payment almost in half, down to around $700 a month.

The money was immediately sucked up. Scott expects his salary at the hospital to be frozen because of Medicare cuts. And the couple keeps postponing maintenance on the family’s 30-year-old home.

The air conditioner stopped working four years ago. The dishwasher is busted, too. The roof is missing a few shingles and leaks in a heavy storm. Last month, Scott installed a hand-me-down cooktop given them by a friend who remodeled her kitchen.

Now the girls are developing an interest in music. Taylor is taking guitar lessons. Emily plays trumpet in the school band, which cost $100 to service last summer. Charlie has taken up the clarinet. The Johnsons bought her a used one from Craigslist but then had to buy reeds at $12 a box. Emily yearns to start track, which has an $85 registration fee.

Robin and Scott are paid on alternating weeks, so every week the Johnsons sit down and discuss which bills take priority. The rhythm of the household rises and falls with the approach of payday.

“You can feel the tension,” Taylor said of her awareness that her dad is worried about expenses. “It eases up when he gets a paycheck.”

Taylor is starting to think about college.

“It scares me,” she said of the future, adding that she may have to attend community college to save money.

Her father regrets not getting his degree and is determined that his daughter will attend college. But he hasn’t set money aside. “I have my retirement account that I can tap for her college,” he said.

Robin’s advice to her daughters: Study hard enough to get scholarships, because we can’t help.

In his State of the Union address this year, Obama called for better job training at community colleges to help middle-class families get a leg up. But training is often a disappointment. In the Washington Post-Miller Center poll, three in four people who said they had acquired new skills through job training also said it hadn’t made any difference.

So far, the extra education and training Robin and Scott have gotten hasn’t done much. Scott paid $3,500 out of his own pocket for a class in network security, but his employer didn’t upgrade to a system that required his knowledge. Robin is looking for a job in which she can use her accounting degree.

Scott has contemplated looking for a new job in Northern Virginia, where the pay scale is higher. But the commute would cut into family time. After attending school across the street, one of Robin’s daughters swings by Scott’s office to do her homework until he can drive her home.

“A lot of what’s kept me [here] is the convenience of being close to the family in the day, if something happens,” he said. “The trade-off isn’t worth it.”

Like his wife, Scott figures that if they could boost their family income to around $150,000, many of their financial cares would ease. For now, Robin fantasizes about winning the lottery. Not Scott.

“I don’t care about being rich,” he said. “I’d rather just be comfortable and not have to struggle every month.”

Peyton M. Craighill contributed to this report.