WASHINGTON — Calls to boycott United Airlines are growing as outrage over the company’s violent ‘‘re-accommodation’’ of an elderly doctor enters its second day.

While the incident has tapped into a deep vein of customer frustration with the practices of United and other airlines, many passengers promising a boycott may find flying without United to be harder than they’re expecting — and in some areas, nearly impossible.

Thanks in part to a rash of airline mergers and consolidations in recent years, major airports are increasingly becoming one or two-carrier affairs. Today, United commands over 50 percent of the market share in some places where it served fewer than 5 percent of air travelers ten years ago.

United could be the cheapest option serving a major air route. Or, it could be the only option. And that makes a boycott complicated.

United tops 50 percent market share in Houston and Newark. At other major hubs, including Dulles, San Francisco, Denver, and O’Hare, anywhere from 28 to 43 percent of passengers are flying United flights.

The airline’s 2010 merger with Continental has allowed it to greatly increase its market share in certain regions. For instance, at Houston and Newark United’s share of passenger traffic exploded from less than 5 percent in 2006 to over 50 percent last year. That’s primarily due to United’s takeover of Continental’s business.

The Department of Justice recently put the kibosh on United’s plans to expand even further at Newark. According to the DOJ’s complaint, ‘‘the enhancement of United’s dominant position would subject air-travel passengers at Newark — who already pay some of the highest fares in the nation — to higher fares and fewer choices.’’

Still, consolidation is a fact of life, meaning that many American airports are dominated by just one or two carriers. All this concentration means that boycotting an airline is a potentially expensive proposition, particularly if your home airport is one of that airline’s major hubs.

Competing airline flights between the same airports can differ radically in terms of price — and not much else. Aggregators like Google and Expedia have made the ticket price the primary factor distinguishing between competing airline services. At the price points most consumers are willing to pay, the flight experience is essentially identical across the major air carriers: cramped quarters, headaches and delays, a litany of TSA indignities before you board.

For an airline boycott to work, in other words, customers would have to be really mad — mad enough to actually make a financial sacrifice.

WHERE ITS HARDEST TO AVOID THE AIRLINE

Here are the major airports where United accounts for more than 10 percent of total passenger traffic:

Houston51.54%

Newark50.49%

Washington Dulles43.25%

San Francisco37.96%

Chicago O’Hare30.87%

Denver28.72%

Los Angeles14.88%

Boston12.28%

San Diego11.95%

Orlando10.26%

Portland10.15%

SOURCE: Department of Transportation