The all-new 2016 Toyota Tacoma has begun arriving at U.S. dealers. Image source: Toyota

Toyota (NYSE:TM) has an all-new version of its midsize Tacoma pickup for 2016. And it plans to make sure that everyone knows about it.

The company is launching a major new ad campaign for its popular pickup, including new television ads that will debut on Monday.  Toyota executives told Automotive News that the campaign will be its biggest advertising push for the Tacoma in a decade.

The Tacoma is still the leader of the small-but-important midsize-pickup segment. But Toyota's decision to give the Tacoma a big advertising push suggests it's starting to get worried about a competitor that's coming up fast in its rearview mirror.

GM's new midsize trucks have found a lot of buyers very quickly
That competitor, General Motors (NYSE:GM) hadn't been a big player in midsize pickups for a while when it launched its all-new Chevrolet Colorado and GMC Canyon last year. But that has changed in a hurry.

According to R.L. Polk figures released this week by GM, its two new midsize trucks captured 31% of the U.S. market for midsize pickups this year through July. (The Tacoma has about half; Nissan's Frontier has most of the rest.)

The Chevrolet Colorado adds a new diesel-powered version for 2016. Image source: General Motors

The Colorado and Canyon are impressive entrants. GM has marketed them as lifestyle trucks -- personal vehicles -- rather than as miniature work trucks. That's a page from the Tacoma's playbook, and it has worked out well for GM.

The two are available with several appealing options packages, which add luxury touches and have helped keep average transaction prices strong. Reviews have been extremely positive -- the Colorado was Motor Trend's Truck of the Year for 2015 -- with the trucks being praised for their quiet rides and good handling on- and off-road. 

The formula has found plenty of interested customers. Demand is very high: The Colorado "turned" in a very short 21 days in September, GM said, indicating that dealers are selling the trucks not long after they're delivered. The factory that makes the twins is now working on three shifts, essentially around the clock. 

GM isn't standing still, either: New diesel-powered versions will begin arriving at dealers in a few months.

That's why Toyota is making a big push for the new Tacoma.

Despite pressure from GM, the Tacoma is still selling well
Toyota first unveiled the all-new 2016 Tacoma in January. Designed in Michigan, the new Tacoma doesn't look a whole lot different from the outgoing model. But Toyota made a slew of changes to its popular midsize pickup, hoping to keep it ahead of rivals like the GM twins.

The new model is in production, but supplies are still tight. Right now, Toyota dealers are still selling down the last of the outgoing 2015 Tacoma models alongside the first of the redesigned 2016s.

The all-new Tacoma retains the off-road abilities that have drawn buyers for years. Image source: Toyota

Despite the new competitive threat from GM, Tacoma sales have been quite good this year. Sales dipped in September -- possibly because of short supplies -- but total Tacoma sales are up 16.5% this year through September.

The company had a 24-day supply of Tacomas as of the end of September, which is very short. (60-90 days is typical.) The short supply might be partly due to the changeover to the new model. But it also suggests that demand is strong. Toyota executives hinted on Thursday that they might be able to sell more of the new ones if they can increase production.

More Tacoma rivals are on the way
For a long time, Toyota had this market segment more or less to itself, with Nissan's entry a distant second. GM's entries have changed that quickly, and they've drawn the notice of other rivals: Ford is expected to launch an all-new version of its Ranger midsize pickup here in a couple of years, while Fiat Chrysler is preparing a new midsize pickup based on the next-generation Jeep Wrangler.

For the all-new Tacoma, the pressure is building. Will Toyota's big ad push help the Tacoma hang on to its sales lead? We'll find out.

John Rosevear owns shares of -- and The Motley Fool recommends -- Ford and General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.