In an aggressive response to Southwest Airlines' decision to begin service from the Twin Cities, Northwest Airlines said Wednesday it would expand its flights to Chicago and match Southwest's low fares.

Northwest will offer competitive prices on Southwest's connecting routes from Chicago. Southwest, which now carries more fliers domestically than any other airline, connects to nearly 50 cities from Chicago's Midway airport.

"We'll have 10 or more trips to both Midway and O'Hare at competitive prices" on airplanes that include both first-class and coach seating, said Tim Griffin, Northwest's executive vice president of marketing and distribution. That's an increase of four to five nonstop flights daily, with some on bigger airplanes than are used now.

Northwest also will be competing with Southwest on connecting routes.

"We always look at the full offerings of competitors, both their nonstop participation in our markets and their connect participation, and factor that into prices that we will offer," Griffin said. "And we'll do so this time."

Southwest CEO Gary Kelly said in an interview in late September, when the airline announced expansion to the Twin Cities, that he'd offer at least 8 to 10 daily departures to Midway beginning in March.

Northwest, United and American offer nonstop service between the Twin Cities and Chicago, but no low-fare carrier has been flying the route since AirTran Airways dropped its service to Midway in May.

Southwest, which will operate 137-seat Boeing 737s, plans to release its schedule and fares on Nov. 6.

Griffin said Wednesday that Northwest stands a good chance of holding on to its customers.

"The big three criteria in customer decisionmaking are schedule, price and frequent-flier programs," Griffin said.

Southwest spokesman Chris Mainz said Wednesday that the airline is known for stimulating traffic and decreasing fares when it enters a market. "It's a classic example of what the Department of Transportation has coined 'the Southwest Effect,' and it appears that Minneapolis-St. Paul is already seeing this phenomenon in action."

Liz Fedor • 612-673-7709