On Wednesday, No. 3 U.S. airline United Continental (UAL 4.98%) significantly expanded its presence in the Hawaiian market. It will operate as many as 11 extra daily roundtrips from the mainland to Hawaii, following through on a plan announced back in June. Most of these flights connect Los Angeles and San Francisco to secondary destinations within Hawaii.

In the short run, this may be a profitable move. Demand for travel to Hawaii has been strong recently, while seat supply has been roughly flat during 2017.

A United Airlines jet on a runway

United Airlines recently added more flights to Hawaii. Image source: United Airlines.

However, the good times may not last long. Other airlines, including Alaska Air (ALK 1.16%) and Hawaiian Holdings (HA 0.15%), also plan to grow in Hawaii's secondary markets. Furthermore, Southwest Airlines (LUV 1.19%) is gearing up to begin flights to Hawaii in late 2018 or 2019. This rebound in competition could undermine United's profitability in Hawaii.

Hawaii is an attractive market

While much of the domestic air travel market has suffered from overcapacity during the past two years, this ultra-competitive environment didn't extend to mainland-Hawaii routes. In 2016, the number of air seats to Hawaii from the rest of the U.S. increased just 1.1%. During the first 10 months of 2017, seat growth was even slower.

This has contributed to solid unit revenue trends for flights to Hawaii. Hawaiian Airlines' recent unit revenue results highlight the strength of this market. Revenue per available seat mile (RASM) increased 2% at Hawaiian in 2016. Through the first three quarters of 2017, the carrier's RASM has surged 7.4%.

In both years, Hawaiian Airlines' routes to the mainland have been the biggest driver of this RASM growth. While other airlines' unit revenue trends in Hawaii probably haven't been quite as impressive, there's clearly plenty of demand to support capacity additions.

A Hawaiian Airlines A321neo on the tarmac

Hawaiian Airlines has posted strong RASM growth in 2016 and 2017. Image source: Hawaiian Airlines.

United Continental has been facing unit revenue pressure across much of its route network lately. Thus, it's understandable that the idea of growing in Hawaii seemed attractive to management.

Competition is rising in Hawaii's secondary markets

Unfortunately for United, it isn't the only airline looking to cash in on strong demand for Hawaii travel. Just last week, Alaska Air subsidiary Virgin America began flying from San Francisco to Kona, giving United Airlines its first nonstop competition on that route. This is one of the routes where United is adding flights starting this week.

As Alaska Air solidifies its market position in California and grows its frequent flyer base there, it will probably add more flights to Hawaii from San Francisco and Los Angeles. That will inject even more competition in the markets where United is currently expanding.

Hawaiian Airlines has even bigger ambitions. Until recently, it has routed the vast majority of its flights through Honolulu, because all of its long-haul planes were widebodies with more than 250 seats. However, it recently began adding the state-of-the art 189-seat A321neo to its fleet.

Hawaiian has already announced four new routes from the West Coast to secondary destinations in Hawaii. It is also using its fleet growth to add flights from Los Angeles and San Francisco to Hawaii, in direct competition with United Airlines. It is likely to continue adding new West Coast-Hawaii routes at a steady pace for the next few years.

The arrival of Southwest Airlines could force United to retrench

While the growth of Alaska and Hawaiian will put some pressure on United, neither of those carriers is likely to carry much connecting traffic to Hawaii. By contrast, legacy carriers like United rely primarily on connecting traffic to keep their planes full.

However, Southwest Airlines could scoop up some of this connecting traffic when it starts flying to Hawaii. As a result, while airline pundits have mainly focused on how Southwest's entry into the market will impact Hawaiian Airlines, United Continental's Hawaii flights could be more at risk of cannibalization.

To be sure, Southwest doesn't fly to as many cities as United. But from Oakland, Los Angeles, and San Diego -- its three most logical gateway cities for Hawaii service -- Southwest Airlines flies nonstop to midsize cities like Albuquerque, Boise, Reno, Spokane, and Tucson. Southwest's low cost structure will allow it to undercut United on fares for connecting itineraries from these cities to Hawaii.

Even if United can retain a strong share of the market for travel from very small cities to Hawaii, that's not a big enough demand pool to support its expanded mainland-Hawaii schedule. Instead, it will be forced to fight tooth and nail for customers with rivals that offer better service and have lower cost structures. That hardly sounds like a recipe for long-term success.