GEOFF BENNETT: In one week, the contract between 340,000 unionized UPS workers and one of the largest package delivery companies in the world expires.

Workers have overwhelmingly authorized a strike and say they're ready to walk if their union, the Teamsters, and UPS can't reach a deal.

The labor dispute could lead to the largest strike in U.S. history against a single employer and cause massive economic disruption in the shipping industry and beyond.

Stephanie Sy is back with a report from Los Angeles on how the battle lines are being drawn.

SEAN O'BRIEN, General President, Teamsters: Are we ready to fight!

AUDIENCE: Yes!

SEAN O'BRIEN: Are we ready to fight!

AUDIENCE: Yes!

STEPHANIE SY: At an early-morning rally outside a UPS facility, the leader of the Teamsters, Sean O'Brien, didn't pull any punches in the city of angels.

SEAN O'BRIEN: We are going to be the example on how it is to fight and take on a schoolyard bully.

PROTESTER: No contract!

PROTESTERS: No work!

STEPHANIE SY: Before talks broke down in early July, the union said many issues had already been resolved with the company.

UPS agreed to end a two-tiered wage system for part-time drivers, new overtime rules, and improvements to keep drivers safe from extreme heat, including a commitment for A.C. in new package delivery vehicles and putting fans in existing trucks.

What are the remaining sticking points in this negotiation?

SEAN O'BRIEN: Economics, completely economics.

We've got 95 percent of the contract negotiated, all favorable for our members.

No concessions.

And UPS just is balking at rewarding the people that make them the success that they are.

STEPHANIE SY: Teamsters point to record UPS operating profits of $13.1 billion in 2022 and $12.8 billion in 2021, when home deliveries were an essential service.

KRIS HARO, UPS Employee: We have been working so hard for the last three years during COVID.

STEPHANIE SY: Kris Haro is a package delivery driver who has worked for UPS for seven years.

KRIS HARO: Nobody wants to go on a strike.

Nobody wants to stop working because, everybody needs UPS.

But at, the same time, look what's happening with the company, making all these profits, and we're not getting none of the piece of the cake.

It's not like we want everything.

We just want what we deserve.

STEPHANIE SY: UPS argues that its union employees are well-compensated, with health care benefits for full and part-time workers, and full-time delivery drivers earning $95,000 a year, on average.

SEAN O'BRIEN: They work 60 to 65 hours to make that money.

That's all overtime.

They make a lot of money, but they earn it.

But they don't tell you about the part-timers, when they starting at $16 per hour.

STEPHANIE SY: More than half of UPS' unionized work force is part-time, including package handlers and sorters in the warehouses.

While UPS says it presented the union with a historic economic proposal, union leaders say wages for part-timers are too low, given the hardships of the job.

Glynis Sims is a part-time driver.

GLYNIS SIMS, UPS Employee: I think everybody should get a fair chance to reach top pay and get paid across the board equally.

You know, so if we have to stand up together, then I'm with it.

STEPHANIE SY: Teamsters have been holding rallies like this around the country and also performing practice pickets.

They insist they will not negotiate beyond the July 31 deadline.

A spokesperson for UPS declined an interview request from the "NewsHour," but in a statement said: "We need to work quickly to finalize a fair deal that provides certainty for our customers, our employees and businesses across the country.

We started these negotiations prepared to increase the already industry-leading pay and benefits we provide our full- and part-time union employees and are committed to reaching an agreement that will do just that."

The last time UPS workers went on strike in 1997, the 15-day action cost the company $850 million.

Today, UPS handles about a quarter of all parcel deliveries in the U.S., a 10-day strike could cost the economy more than $7 billion, according to one estimate.

A strike would also affect millions of customers, like small business owner Alex Dettman.

He sells antiques and collectibles on eBay and Etsy from his home in Minneapolis.

ALEX DETTMAN, Etsy and eBay Seller: The larger items, the heavier, bulkier boxes that I send, really have to go UPS.

You can ship them through the post office, but it can be twice as much, if not more.

STEPHANIE SY: While Dettman is sympathetic to the UPS union, he's concerned what a strike will mean for his business.

ALEX DETTMAN: I think people will worry about all of the carriers.

You know, when is my item going to arrive?

Is it going to get lost somewhere?

And they will just say, forget it.

That means maybe a bad August for me and lots of other Etsy sellers.

STEPHANIE SY: Supply chain experts say UPS risks losing market share to its competitors.

In advance of the strike, FedEx is encouraging UPS customers to switch, and the U.S.

Postal Service says it has the capacity to handle what is given to it.

KENT WONG, Director, UCLA Labor Center: The reality is that UPS drivers and the Teamsters union have tremendous leverage.

STEPHANIE SY: Kent Wong is the director of the UCLA labor center.

KENT WONG: The whole point of a strike is disruption, to extract economic harm to the company in order for a more favorable deal at the bargaining table.

STEPHANIE SY: Wong points out that public support for unions is higher than it's been in decades and strike strategies have worked recently, including for graduate student workers and L.A. school employees.

KENT WONG: When workers organize, when they take collective action, they generally win.

STEPHANIE SY: Organizers are betting the momentum continues.

On social media, the bold union actions from Hollywood to hotels are hashtagged #hotlaborsummer.

And unions are showing solidarity.

At the rally in L.A., there were more screenwriters than UPS workers.

Zev Frank, a member of the Writers Guild of America, has been on strike since may.

ZEV FRANK, Writers Guild of America Member: The same kinds of grievances that actors have, that writers have, that Teamsters in Hollywood have, that Teamsters at UPS have, there's a common through line.

There has been a massive transfer of wealth upwards in this country.

And the only way we're going to put an end to it is by organizing and coming out for one another in these displays of solidarity.

STEPHANIE SY: Given the stakes of a possible strike, hundreds of business groups have urged the Biden administration to intervene, as it did recently to avert a rail strike.

Union chief Sean O'Brien firmly rejects the suggestion.

SEAN O'BRIEN: We will settle our problem one way or the other.

We try to be diplomatic.

We try to be reasonable, but, sometimes, people don't want to listen.

STEPHANIE SY: In front of the crowd, the tone is far from diplomatic.

SEAN O'BRIEN: If you want to fight, put your helmets on and buckle your chinstraps.

It's a full-contact sport.

(CHEERING AND APPLAUSE) STEPHANIE SY: At least some of the UPS workers sounded less combative.

KRIS HARO: Don't take me wrong.

This is a great company.

We get paid really well.

We get really, really good benefits, everything.

I love the company, but we deserve more for the hard work we've been doing for a long time.

STEPHANIE SY: With one week until the deadline, negotiations between Teamsters and UPS are resuming tomorrow, a sign a deal may still be reached.

For the "PBS NewsHour," I'm Stephanie Sy in Los Angeles.