The air conditioning on the Route 5 bus one recent rainy morning seemed negligible at best. The windows steamed up, creating a kind of communal cocoon for sodden passengers on their way to work, school, home, doctors' appointments — or perhaps no place in particular.

Metro Transit's Route 5, which links Brooklyn Center to the Mall of America through north, south and downtown Minneapolis, is the busiest transit corridor in Minnesota and serves more than 4 million passengers a year.

But a planned upgrade to more-efficient and comfortable rapid bus service called the D-Line — offering a light-rail passenger experience for a fraction of the cost — is uncertain after the $75 million project failed to secure $35 million from the Minnesota Legislature this year.

While millions were allocated in this year's bonding bill for highway improvements across the state, public transit received nothing — an omission Gov. Mark Dayton called "ignorant and irresponsible." Republican legislators say there was only so much money to go around.

But the result, said Sen. Scott Dibble, DFL-Minneapolis, is that "the D-Line is dead in the water right now. It's not going forward, unless this money is appropriated."

The Metropolitan Council has bet big on rapid bus service to round out the Twin Cities' public transportation system. One route — the A-Line in the east metro — is up and running, and an additional 10 lines are being planned.

Bolstered by the success of the A-Line, which increased bus ridership along Snelling Avenue by a third in its first year, the council is now building the C-Line. Traveling mostly along Penn Avenue between Brooklyn Center and downtown Minneapolis, the C-Line is fully funded.

Despite uncertainty over funding, the Met Council continues to work on the D-Line. Some 40 stations are slated for consideration by the regional planning body this month.

"As we move forward with planning for the D Line, there is no clear solution for how to fully fund the project," said Met Council Chair Alene Tchourumoff, in an e-mail. "We will be working with our partners and the many supporters of the D Line to seek ways in the coming year to secure the remaining funding.

"This project," she added, "is too important to leave on the table."

Life aboard the 5

The bitter divide over transit funding seemed far removed on a recent trip aboard the Route 5. Most passengers interviewed weren't aware of the planned D-Line, and some worried bus service would be taken away.

"If it's faster and there's better buses, that's a good thing, right?" said Ashley Canada, as she rode to her job at the Mall of America.

Rapid buses generally arrive every 10 minutes, and fares are collected before passengers board the bus, which is one reason why they're 20 percent faster than existing service. The current Route 5 can take 90 minutes to travel from one end to the other.

Rapid bus stations are brightly lit and platforms have a raised curb, making buses easier to board. Buses stop in traffic, eliminating the need to pull in and out. They also are equipped with signal priority at certain intersections, meaning bus drivers can request early or extended green lights so they can keep moving.

The Route 5 was chosen for the upgrade because "it's our workhorse, it's our busiest route," said Charles Carlson, Metro Transit's director of bus rapid transit projects. "It's an essential lifeline and critical piece of infrastructure for the 16,000 people who ride it every day."

During rush hour, Metro Transit officials say, Route 5 buses make up less than 2 percent of vehicle traffic but carry more than 20 percent of people traveling through the corridor.

"I have taken this route all my life," said Diana Holden, a nursing assistant who lives in south Minneapolis. "It gets busy, but it's reliable."

Bus ridership in the Twin Cities and across the country has declined in recent years, due to low gas prices and the popularity of ride-sharing services such as Uber and Lyft. But regulars on the Route 5 say current service could be improved upon, too.

Over the years, the route has attracted negative attention after some bus drivers were assaulted by passengers. Michael Brown, a Metro Transit Route 5 driver, estimated that about "3 percent" of passengers cause trouble by "playing their music too loud, cussing" and not paying their fares.

Brown said he doesn't worry about his own safety, although some of the buses are slated to be outfitted with plexiglass barriers to protect drivers.

"It's an easy route," he said. "There aren't a lot of turns."

Not everyone is on board with the locations proposed for D-Line stations, especially one slated for 48th Street and Chicago Avenue in south Minneapolis.

Harvey McLain, owner of Turtle Bread restaurant and several buildings in the neighborhood, worries about parking being eliminated to make way for the station. In a letter to neighbors, he said the current bus stop "attracts some bad characters" who are "panhandling and selling and using drugs."

McLain says he supports transit, but he thinks the D-Line stop two blocks away at 46th and Chicago will suffice.

Dibble: Not enough money

Even if the D-Line is built, the Route 5 bus won't go away entirely. It simply would operate less often.

"People using this line are trying to go about their lives — low-income people, young people, seniors, disabled people and choice riders, like millennials who don't want to be tied to a car," Dibble said.

He added that a reliable source of transportation is critical to lifting people out of poverty. But convincing his Republican colleagues of the D Line's attributes has left him frustrated.

"I might as well be talking to my sofa right now," he said.

Rep. Dean Urdahl, a Grove City Republican who chairs the House Capital Investment Committee, said there wasn't enough bonding money to go around for transportation projects this session.

"It comes down to priorities," he said. "We'll take a closer look at transit needs next time."