For their first six seasons as a franchise, the Oakland Raiders lived a nomadic existence, playing home games at such venues as Kezar Stadium, Candlestick Park and Frank Youell Field. In 1966, they moved into the Oakland-Alameda County Coliseum, newly built along the Nimitz Freeway.

“It was like a dream come true walking onto the field,” recalled Tom Flores, the former Raiders quarterback and coach. “It was like, ‘God, we finally have a home.’”

The Raiders, bound for Las Vegas in 2020, will soon bid farewell to the stadium that has hosted them for most of the last 52 years. With no home established for next season, the Raiders may in fact be playing their final game at the Coliseum on Monday night against the Broncos.

A long goodbye, soured already by the Raiders’ poor record this season under returning coach Jon Gruden, accelerated in recent weeks. The city of Oakland on Dec. 11 sued the Raiders and NFL over the team’s impending move. The Raiders responded by withdrawing from a tentative agreement to play in Oakland in 2019.

The resulting uncertainty over the team’s immediate future, shared by players and fans alike, converges Monday with Christmas Eve and the glare of national TV to create what could be a bizarre mood inside a venue long known for its costume-clad fans and the rowdy Black Hole.

“I expect this to be a great atmosphere — probably the best we’ve ever had,” quarterback Derek Carr said last week. “It’s weird to think this could possibly be the last game. I don’t want that, I know I don’t. I know our fans don’t. But the fact that it could possibly be — I think it’s going to be a great atmosphere.”

Chris Dobbins, co-founder of fan group Save Oakland Sports and a longtime Raiders fan, said he isn’t so sure.

“I think it’s going to be very upset,” Dobbins said. “I know a lot of fan groups are passing out signs and doing stuff. There’s also talk that people are going to rush the field after the game — I don’t think that’s going to happen. But I just think everyone’s going to be really mad.”

As their home from 1966-81, and from 1995 to now following a 13-year stint in Los Angeles, the histories of the Raiders and Coliseum are intertwined.

The Raiders, then still of the AFL, reached Super Bowl II in their second season at the Coliseum, losing to the Green Bay Packers. They would reach and lose conference championship games in six of the next eight seasons, breaking through in the 1976 season for their first Super Bowl win.

Raiders in Oakland April 15, 1964: Construction begins on Oakland Coliseum Sept. 18, 1966: Raiders play first game at Coliseum, losing 32-10 to Chiefs Dec. 31, 1967: Host AFL Championship, a 40-7 win over Houston Jan. 4, 1970: Host first of five AFC Championship games, a 17-7 loss to Chiefs Dec. 23, 1981: Lose 23-6 to Bears in final game before moving to Los Angeles Sept. 3, 1995: Beat Chargers 17-7 in return to Oakland after reaching deal to renovate Coliseum, including “Mt. Davis” expansion Jan. 19, 2003: Beat Titans 41-24 in last home playoff appearance Dec. 24, 2018: Host Broncos in what is possibly final game in Oakland 173-126-3 all-time regular-season record at Coliseum 14-3 playoff record at Coliseum 88-24-3 regular-season record at Coliseum before move to L.A. 85-102 regular-season record at Coliseum since return

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By that time, said longtime Oakland Tribune columnist Dave Newhouse, the Raiders’ stadium was becoming a formidable venue for visiting teams.

“You would look out at the stadium from the press box and see nothing but black — everybody was dressed in black,” Newhouse said. “And before they reconfigured the stadium, the stands were right behind the visitors’ bench. Horrible, the things they yelled. It’s an X-rated fan base.”

Newhouse recalled a Tribune columnist once trying to capture the experience — by walking through a Coliseum tailgate wearing a Chiefs jersey.

“It was like trying to cross the German border during World War II,” Newhouse said.

It was much different, though, for a Raider. Flores, who won two Super Bowls as Raiders’ head coach, recalled arriving at the Coliseum for games and hearing fans in the parking lot yell: “Hey, Tom!”

“I would smile and look and see what they were having for barbecue that day,” Flores said. “It was like you were going to a backyard outing.”

Flores was coach and Jim Plunkett the quarterback when the Raiders left Oakland the first time, moving to Los Angeles before the 1982 season. Plunkett, a Bay Area native, said he believes the fan base in Oakland “deserves a team,” but acknowledged the Coliseum — the only stadium that currently hosts an NFL and MLB team — is “run-down.”

“It’s sad for (Oakland fans),” Plunkett said. “They’re going to lose the Warriors. I don’t know exactly what’s going to happen with the A’s. And Oakland identifies with these professional sports teams. I wish (the Raiders) would be able to stay. But that being said, Vegas has stepped up and is going to build them a beautiful new stadium.”

The domed $1.8 billion stadium being built for the Raiders in Las Vegas included $750 million in public funding. The NFL approved the team’s move in March 2017 — but that left three seasons to play as the stadium was built. Gruden, who coached the Raiders from 1998-2001, returned this year vowing to “help deliver the best football team we can for the people here in Oakland.” But the Raiders go into Monday night’s game at 3-11, tied for the worst record in the NFL.

Between the team’s two departures, Newhouse said, Raiders fans in the Bay Area have “been abused almost beyond recognition.”

“There are no scars or bruises on them,” Newhouse said. “But internally, they must be a basket case.”

Rich Gannon, the Raiders’ quarterback from 1999-2004, deemed it “a special fan base that’s fiercely loyal.”

“I hate to see it happen,” Gannon said. “I am excited about the team going to Vegas, because I think it’ll provide better opportunities for them in terms of resources they’re going to have. ... They’ll get a brand-new facility, a state-of-the-art training center and all those types of things.

”But I thought it was going to be another year, anyway. So I think everyone’s disappointed.”

Over the years, the Coliseum’s flaws could verge on humorous. Amy Trask, the former Raiders’ CEO and current CBS Sports analyst, remembered taking a phone call during a pregame power outage from a visiting team official asking her to: “Let me know when y’all have some power in the building.” Another time, the hot water went out in the visiting team’s locker room.

“Their president said to me, ‘We have no hot water, so your guys shouldn’t be able to shower either,’” Trask said.

“Visiting teams did not like our accommodations. They didn’t like the locker rooms, they didn’t like the power outages and the water outages. And we were OK with that.”

To some, the building is endearing. Gruden last week called it a “real football stadium. It’s dirt, grass. It has tradition.” Broncos offensive tackle Garret Bolles, meanwhile, reportedly declared the Coliseum “a dump.” Carr, calling it “a special place,” said he has heard all the complaints.

“People talking trash about it through the media, saying our machines are broke or saying the field is terrible, or the locker room stinks, and all of that,” Carr said. “And I just laugh. Because it’s home to me, you know?”

Matt Kawahara is a San Francisco Chronicle staff writer. Email: mkawahara@sfchronicle.com Twitter: @matthewkawahar