They came to Nationals Park in historic numbers, bringing with them the intensity of a fan base that expects greatness.

Their red Strasburg and Harper jerseys filled Metro trains as far out as Virginia almost two hours before game time. Their cheers at each player’s introduction nearly drowned out the boom of fireworks. Their high-pitched wails to A-ha’s “Take On Me” — their team’s unusual anthem — echoed in perfect unison.

“It’s amazing,” said 29-year-old Chuck Pollard, gazing around at the stadium that was still mostly full in the ninth inning. “D.C. definitely likes a winner.”

Added his fiancee, 28-year-old Stephanie Tyson: “It feels like everybody’s all in.”

If opening day 2013 was any indication, baseball, which returned to the District in 2005, has finally permeated the fabric of this city and region. The announced attendance of 45,274 set a record for a regular-season game. Mike Corum, 62, paid $40 to a scalper for a spot in the standing-room-only crowd. He said he felt as if he got a bargain.

“I’d have paid $50,” Corum said, standing on his toes to peer between the people in front of him. “I like this, because the hope is so high here.”

“Hopeful,” though, does not seem a strong enough adjective to describe the mood of fans toward their 2013 Nationals. Those at opening day 2012 were “optimistic” about a promising young team, said Dave Tyahla, 46, a lobbyist from Arlington. “This year,” he said, “the place is just buzzing. The whole city’s going nuts about this.”

Call it the Sports Illustrated effect: The magazine, after all, picked the Nationals to win the World Series. Blame it on Davey Johnson, the Nationals’ manager, who has branded this year’s slogan “World Series or bust.” For many, going to a Nationals game is no longer a diversion on a mild spring afternoon. It’s something that requires commitment, especially on opening day.

“It’s kind of like a religion,” said Mike Mullen, 53, who came to the ballpark dressed in a suit, having walked over from work at nearby National Defense University. “You have to come commune with your fellow fans.”

One man, who would give his name only as Terry, said he was skipping out on work for a few hours to come to the game Monday. He reasoned that “any baseball game’s worth missing work,” and opening day during what might be a World Series year was particularly significant.

“This is incredible,” he said. “The place is packed.”

Some fans fretted over the massive expectations — noting that even with 98 regular season wins last year, the team suffered a painful ouster in the playoffs.

John Fourkas, 49, who came to opening day with his wife, Amy, and son, Austen, said he thinks regularly about the final strike that he thinks umpires were unwilling to call in that playoff series. But that, he said, does not dampen his hopes for this season.

“In one sense, I feel like, ‘Oh boy, they’ve got to climb the hill all over again,’ ” he said, watching batting practice from his seat three rows above the field. “But in another sense, they arrived a year early.”

Added Amy Fourkas: “I think they’re going all the way, but I’ve been saying that every year.”

Mark Schwanbeck, 61, said he thinks the team will “be a contender all the way through if they stay healthy,” and it could remain that way for years to come.

“I think it’s obviously the talent, but the age of the talent — all of these guys are entering into or right at their primes,” Schwanbeck said. “Last year was phenomenal, and we have expectations that this year will be even more phenomenal.”

On Monday, he and more than 45,000 others were not disappointed. Bryce Harper homered twice, Stephen Strasburg pitched seven shutout innings and the Nats did nothing to temper the talk of postseason accolades.

“If this is going to be an MVP season, that’s a nice way to start it,” Charles Kitchen, 27, said of Harper as he watched the game’s final strike. “I think Davey Johnson was right when he said ‘World Series or bust.’ ”