On a day when the Indianapolis Colts clinched home field advantage throughout the AFC playoffs and confirmed yet again that they’re the best team in football, it was the NFC playoff race that became most intriguing, as two division leaders lost while the teams in second place won.

In the NFC North, the Chicago Bears’ eight-game winning streak came to an end while the Minnesota Vikings continued their stunning turnaround and improved to 8-5 with their sixth straight win. With Chicago only a game ahead of Minnesota in the standings, the teams’ meeting on January 1 at the Metrodome is shaping up to be the biggest game of the final week of the season.

Chicago’s 21-9 loss to the Pittsburgh Steelers yesterday revealed that when the great Chicago defense doesn’t bring it’s A-game, the offense, led by rookie quarterback Kyle Orton, can’t compensate. By the time the Steelers had established a 21-3 lead on a Ben Roethlisberger touchdown pass and two TD runs by Jerome Bettis, the game was all but over. The Bears, who hadn’t given up 21 points since Week 3, folded up when forced to come from behind, gaining just 283 total yards, including 83 on the ground. In the process, the Bears betrayed some weaknesses that could cost them down the stretch.

Chicago has had the best defense in the NFL this year, but Pittsburgh showed that its brand of power running is the perfect antidote to Chicago coach Lovie Smith’s preference for small and fast defensive players. Bettis, who at 255 pounds is bigger than four of the five linebackers on Chicago’s roster, carried 17 times for 101 powerful yards. Willie Parker added 21 carries for 68 yards, often running behind guard Jeff Hartings, who paved the way for many of the yards by blocking Chicago middle linebacker Brian Urlacher.

Chicago’s safeties usually specialize in playing close to the line of scrimmage and preventing opposing running backs from catching passes out of the backfield. But with starting safeties Chris Harris and Mike Brown out with leg injuries, Pittsburgh was able to dictate the style of play. Roethlisberger had only 175 passing yards, but 79 of them came on passes to running backs.

The Bears now have to worry about staying ahead of the revitalized Vikings, whose sudden surge has coincided with the rebirth of 37-year-old quarterback Brad Johnson, who had another efficient day yesterday, completing passes to nine different receivers and going without a turnover for the second straight week.

Johnson has certainly given Minnesota a new breath of life since the season-ending injury to Daunte Culpepper, it’s a general rule that quarterbacks take too much blame when their team loses and too much credit when it wins. Johnson’s play shouldn’t overshadow Minnesota’s defense, which harassed Rams rookie Ryan Fitzpatrick all day, intercepting him five times. That defense could make Minnesota the favorite when the Bears visit the Metrodome – the last thing the Bears want to face is a secondary that picks on rookie quarterbacks.

At 8-5, Pittsburgh probably can’t catch first-place Cincinnati in the AFC North.But the Steelers are fighting for a wild-card berth, which makes their visit to Minnesota next week a crucial game.

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In the NFC South, cornerback Ronde Barber led a Buccaneers defense that is still one of the league’s best to a 20-10 victory over the Carolina Panthers and a share of first place. Barber recorded an interception and a sack, and Tampa Bay’s front seven held Carolina’s two running backs, Stephen Davis and DeShaun Foster, to 69 yards on 18 carries.

Tampa Bay’s offense played the ball control style that teams with good defenses always try to play in December. Chris Simms threw short passes almost exclusively, completing 20 of 27 for 138 yards. Rookie running back Cadillac Williams is healthy again and gained 112 yards on 29 carries, including an impressive 10-yard touchdown in the fourth quarter to seal the game. Despite stacking the line of scrimmage in anticipation of that run, Williams had no trouble running past the Panthers’ weary linebackers. Plays like that will have Carolina’s coaching staff questioning whether they can win in January.

To be a real playoff threat, Carolina needs to find an identity to its running game. In the past, Davis has had the kind of steady, consistent style that head coach John Fox and offensive coordinator Dan Henning prefer. But this year Davis, 31, has been supplanted as the starter by Foster and has consequently seen only 20 carries in the past four weeks. Foster, however, is maddeningly inconsistent, mixing in games like last week’s 180-yard performance in a win over Atlanta with games like yesterday’s, when he gained 28 yards on his first three touches and then gained 20 yards on his final 13.

Both the Panthers and Bucs are 9-4, and the team that doesn’t win the South will probably win a wild card. But yesterday’s game gave Tampa Bay the tiebreaker advantage, and the Bucs look like a team built to win in the playoffs while the Panthers look like a team in disarray.

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Another second-place NFC team, the Dallas Cowboys, pulled out a last minute win against the Kansas City Chiefs, but Dallas stayed a game behind in the NFC East with the Giants’ overtime victory.

Drew Bledsoe had his best game in a Cowboys uniform, completing 22 of 34 passes for 332 yards, three touchdowns, and no interceptions. But Dallas was again let down by its run defense, which allowed Kansas City’s Larry Johnson to rush for 143 yards and three touchdowns, which can’t please Bill Parcells as the playoffs approach. But the Cowboys will go down to the wire fighting with Minnesota and Atlanta for the final NFC wild-card spot.

The Chiefs’ loss, plus a surprising loss to the Miami Dolphins by the San Diego Chargers, essentially handed the AFC West to the Denver Broncos, who, at 10-3, will now battle Cincinnati for a first-round bye in the AFC playoffs. Several races will be decided in the final three weeks, but one thing is certain: The Colts are the team to beat.

Mr. Smith is a writer for the statistical Web site FootballOutsiders.com.