RICHMOND, Va. — After seven months and 26 regular-season races, NASCAR has hit that part of the year that really matters. The start of the playoffs signifies a shift in attitude and aggression.If there were any doubt of that, Kevin Harvick posted a warning about his mood for the next 10 weeks. It was a video of an angry bull charging into the grandstands.And so the Sprint Cup's Chase for the Championship begins Sunday at Chicagoland Speedway with 16 drivers laser-focused on the big prize. It's 10 races, three rounds of elimination, and every man for himself.So don't be surprised if tempers boil over, paint is traded on the track, angry words are exchanged. It has already started, to a degree. Tony Stewart has intentionally wrecked drivers in consecutive races, including Saturday night's regular-season finale with contact that officially ended Ryan Newman's shot, slim though it might have been, at making the Chase.A frustrated Newman called his former boss "bipolar" and said Stewart had anger issues. Stewart was nonchalant, saying Newman had it coming after running into him three times during the Federated Auto Parts 400 on the 0.75-mile track."How many times does a guy get a free pass," Stewart told NBC after the race. "Three times? That's two more times than I normally let somebody run into me."Smoke has a well-known temper, but the Chase and this format can bring out the worst in even the most mild-mannered drivers. Matt Kenseth tackled Brad Keselowski in a very un-Kenseth type attack in 2014, then earned a NASCAR suspension last year for an intentional crash that ruined Joey Logano's title chances.Denny Hamlin, who won Saturday, said the jockeying for the final Chase spots led to some recent aggression. He also speculated the length of the season could be a contributor, especially for drivers not battling for the title.As for the Chase drivers? When the field will be trimmed by four at the end of each round, Hamlin expects to see tense racing."As guys get eliminated, it could definitely ramp back up again," he said.For some, like Chris Buescher, just getting into the 16-driver field was the prize.Last year's Xfinity series champion made the Chase by winning a rain-shortened race at Pocono, but he doesn't want to settle for a "happy to be here" attitude."We look at this first round, and we want to make it past that round. We want to move through the Chase," Buescher said. "Then we can re-evaluate from there. If we can keep going farther and improve our program each and every weekend, that's always going to be what we're aiming to do."Stewart is also assessing his final Chase appearance before he retires at the end of the year. He slumped his way into the 2011 Chase and said his team didn't even deserve a spot in the field.Then he won the whole thing, his third NASCAR championship."Who knows what's going to happen?" he said Saturday. "All I care about right now is getting ready for Chicago. … This is a stressful 10 weeks and you take it one week at a time. That's what we did in '11, and it worked."