KANSAS CITY, Mo. — For the Seahawks, it is now a season on the brink.

“We can go either way,” Earl Thomas said grimly. “We can run, or we can stay on track. I think we’ll stay on track.”

But now, the track is unforgiving, as grueling as a treadmill at warp speed. The Seahawks have squandered their margin for error, and face an increasingly uphill road to even get a chance to defend their title in the playoffs.

Sunday, the Seahawks let another victory slip out of their hands. Last year, they would have grabbed this game and not let go. They would have squeezed it into submission.

But their vise grip has weakened. When the Seahawks needed to stop Jamaal Charles, they let him wriggle out of their grasp and spring loose for 47 yards. When they needed a short fourth-down conversion, they were stymied. Twice.

Alarmingly, that has become a trend. This 24-20 defeat against the Chiefs at frigid Arrowhead Stadium was the third one this year in which they had a drive at the end with a chance to win, and failed to do so. Their other loss came when St. Louis flummoxed them with a fake punt, and an apparent Richard Sherman fumble recovery was negated by the officials.

Do the math, and it’s one more defeat than they had all last season — a year that is looking more like a highlight reel from the past than a repeatable blueprint. Everyone said it would be hard to recapture the magic, and that reality has proved brutally accurate.

It’s not that they’re being blown out; it’s just that they’re not seizing the chances that dictate success or failure.

Why? You could point to injuries, depth, hunger, strength of schedule, opponents’ motivation — all the usual suspects. Or you can take the mindset that what happened in 2013 was just a unique confluence of talent, karma, luck and desire and say that last year, not this one, was the aberration.

“It’s football, man,” Thomas said. “People are trying to put this weird spell on why this is not happening and why that’s not happening. That’s the way the ball works, man. It’s human nature out there. There’s a lot of factors. People don’t understand that. All we can do is keep fighting.”

But the odds for that fight are getting longer and longer. Arizona looks increasingly dominant in the NFC West, opening up a three-game lead after its victory Sunday. That leaves, for the moment, four teams fighting for two wild-card playoff berths — two of whom, the Packers and Cowboys, are one-up on Seattle. The other, San Francisco, has won two straight despite its soap-opera atmosphere.

The increasing degree of difficulty might play right into the Seahawks’ underdog sensibilities. But now they can’t talk anymore about turning things around at some nebulous point in the future. They have to do it now.

“I reflect back to my rookie year when we had five losses and then won the rest of our games,” said the eternally optimistic Russell Wilson. “For us, we’re just looking for the next opportunity. We have to have that high confidence we normally have and continue to have that swagger we always play with.”

But swagger doesn’t trump missed tackles. High confidence is offset by the continual loss of impact players, from Brandon Mebane and Zach Miller to Max Unger, knocked out of the game Sunday at a critical juncture by an ankle injury.

“We have to get back to ourselves,” said defensive end Cliff Avril. “We have to get back to making plays we know we can make.”

It really is as simple, and as complex, as that. Five times Seattle went into the red zone and scored just two touchdowns. Needing a score to move ahead in the fourth quarter, the Seahawks had first-and-goal at the 9 and didn’t score, and then watched Marshawn Lynch get stopped cold on fourth-and-one from KC’s 36.

“That’s usually where we thrive,” said tight end Luke Willson. “Today it didn’t happen. It was kind of a weird game.”

Weird is a good word to describe Lynch’s decision to spend halftime outdoors on the sideline, where the wind chill was 10 degrees at kickoff, rather than join his teammates in the toasty locker room. But it didn’t detract from another stalwart effort from Lynch, marred only by his no-gain on that vital fourth-down play.

“They really put the play call on our backs as an offensive line, and we have to get that grind,” tackle Russell Okung said. “I definitely have to play better.”

That sentiment of personal accountability was heard throughout the locker room. It will need to manifest itself in a cumulative fashion as the Seahawks prepare for a truly must-win game against Arizona at CenturyLink Field.

“Everything is correctable,” Doug Baldwin said. “You have to get better. We have to be real and tell the truth Monday. It better be real Monday, too. We have to come ready to fix the issues we have. … If we’re going to control our own destiny, we actually have to control our own destiny.”

Destiny, it seems, is a fickle condition.

Larry Stone: 206-464-3146 or lstone@seattletimes.com. On Twitter @StoneLarry

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