(CNN) -- The nation's aviation security chief is sticking to his decision to permit small knives on planes, despite increasing pressure to reverse course.
There was immediate opposition from some pilots and flight attendants to the decision last week by Transportation Security Administration Administrator John Pistole.
Those flight crew members are concerned the decision dangerously relaxes aviation security and sends the wrong message to a flying public eager for more simplicity and efficiency in passenger and bag screening.
But the situation in Washington grew more intense on Monday.
By afternoon, more than 20,000 people had signed a petition to whitehouse.gov calling on the TSA to keep knives off planes.
Moreover, key lawmakers have sought a reversal of the decision, and the biggest airlines are now wondering if the mater was thought through carefully enough.
"Additional discussion is warranted before items that have been banned for more than 11 years are allowed back on aircraft," said the industry's chief trade group, Airlines for America.
One industry leader, Delta Air Lines chief executive Richard Anderson, said previously the decision will add little to aviation security efficiency while raising risks for passengers and crew members.
But Pistole is resolute that the move is correct, that U.S. aviation security has improved since the 2001 al Qaeda hijack attacks on the United States to the point that the likelihood of a similar plot succeeding against a commercial airliner is remote.
"Our intent is to implement the changes on April 25," TSA spokesman David Castelveter told CNN on Monday.
Pistole is expected to defend the decision on Thursday at a House Homeland Security Committee hearing.
Castelveter said the new rule is consistent with Pistole's embrace of Risk-Based Security. This emphasizes detection of liquid explosives and improvised bombs that experts say pose a far greater risk to airline security.
Experts believe hardened cockpit doors and engaged passengers minimize chances that hijackers using small knives would be successful. Bombs, however, can be hard to detect and can get aboard planes in a number of ways, including cargo.
The agency, created in the aftermath of the 9/11 attacks, banned small blades and other sharp objects in response.
Over the past decade, it has added and subtracted items from its prohibited list, permitting small screwdrivers, small scissors, cigarette lighters and matches, and banning large quantities of liquids and gels.
The TSA said the latest decision will bring the United States into basic compliance with international standards. These allow knives shorter than 6 centimeters, or 2.3 inches, aboard aircraft.
Castelveter said Pistole twice considered internal proposals to alter the list without making changes. But in his third review, which included vetting by TSA leadership and security experts, Pistole opted to act.
In response to concerns from Federal Air Marshal Service leadership, Pistole continued excluding small knives that most closely resemble weapons, specifically those with blades that lock in place or have molded hand grips.
He also decided to keep box cutters and razor blades on the prohibited items list because there is "too much emotion associated" with their use in the 9/11 hijackings.
Still, Sen. Charles Schumer, D-New York, and Rep. Ed Markey, D-Massachusetts, both called upon Pistole over the weekend to reconsider his decision.
And on Monday, Rep. Bennie Thompson of Mississippi said the decision appeared to have been made "in a vacuum" and will impact millions of passengers.
Thompson is the senior Democrat on the panel that will hear Pistole's testimony later this week.
Others opposing the change include the Flight Attendants Union Coalition, representing nearly 90,000 flight attendants, and the Coalition of Airline Pilots Associations (CAPA), which represents 22,000 airline pilots.
"We believe the threat is still real and the removal of any layer of security will put crew members and the flying public unnecessarily in harm's way," CAPA President Mike Karn said.
In addition to the small knives, the TSA will allow passengers to carry two golf clubs, toy bats or other sports sticks -- such as ski poles, hockey sticks, lacrosse sticks and pool cues -- aboard in carry-on luggage.
Pistole said in a three-month period last year, the TSA seized 47 small knives from passengers at Los Angeles International Airport alone. But the knives presented little threat to the plane, he said.
"Our greatest concern, the greatest risk, is non-metalic IEDs, whether that's explosive, whether that's (an) electronic initiator or a chemical initiator, whatever that may be. That's what I want security officers to focus on," he said.