In a letter to Los Angeles Mayor Eric Garcetti, Feinstein noted that almost half the flights at the once fast-growing airport have been eliminated, causing gates to shutdown, concessions to leave and significant job losses.
"I have heard deep concerns from many business and community leaders that the decline of Ontario International Airport traffic will have a long lasting, negative regional impact on the Inland Empire," Feinstein wrote.
Since 2007, the number of passengers at Ontario have plummeted from 7.2 million to about 4.3 million in 2012. At the current rate, the level could fall below 4 million this year.
Officials for Los Angeles World Airports, the operator of Ontario, blame the drop on the recession, which forced airlines to move to larger airports, such as Los Angeles International.
Inland Empire officials, who are seeking to gain control of the airport, claim that poor management, severe advertising cutbacks and little progress to lower some of the highest airline costs in the region have significantly contributed to the decline.
They have sued Los Angeles World Airports, claiming the agency has reneged on a longstanding agreement to do its best to develop airline service at the facility.
The lawsuit has been halted temporarily so talks could resume related to a possible transfer of ownership from Los Angeles to the Ontario International Airport Authority.
"We are grateful to Sen. Feinstein for engaging on this vital issue that has united all of Southern California," said Ontario City Councilman Alan Wapner. "We look forward to productive negotiations with Mayor Garcetti."
Feinstein noted that many airports across the nation and California have made significant progress in their recoveries from the economic downturn, even those with challenges similar to Ontario's.
Those airports, she stated, have worked with local business leaders, transit agencies and governments to improve transportation access, reach out to new carriers, waive fees for airlines and offer other incentives to attracts service.
The senator has discussed the airport's continuing decline with Inland Empire mayors, county supervisors and community leaders.
Los Angeles officials say they have worked to reduce airline costs by cutting expenses and staff at Ontario. They also have begun marketing, advertising and incentive programs to try to attract more air service.
Feinstein noted that the recently initiated programs appear to be bearing some fruit.
Despite those programs, airport statistics show that Ontario's passenger volume is down about 8.7% so far this year and additional losses of service are predicted for 2014. Inland Empire officials have asserted in the past that the initiatives by Los Angeles are too little too late.