U.S. Sens. Jeff Flake and John McCain have had two chances in the last week to state their support for the continued existence of the A-10, the mainstay of Tucson’s Davis-Monthan Air Force Base.

They didn’t take either one.

The reasons why aren’t perfectly clear, but chances are they relate to the Air Force’s new, budget-busting F-35 — and in a roundabout way, bring us back to the old Phoenix-Tucson rivalry. Guess who’s winning?

When U.S. Sen. Kelly Ayotte, R-N.H., wrote a letter to the defense secretary and the chairman of the Joint Chiefs of Staff last week, objecting to proposals to retire the A-10, only one member of Arizona’s congressional delegation signed on — U.S. Rep. Ron Barber, the Tucson Democrat who represents D-M.

Democratic U.S. Reps. Raúl Grijalva and Ann Kirkpatrick, who also represent the Tucson area, did not sign the letter.

This week, Ayotte introduced an amendment to the National Defense Authorization Act that would prohibit the Air Force from retiring the A-10 until the F-35, its planned replacement, has shown it is able to fulfill the role of providing close air support to combat forces on the ground. When I asked spokesmen for Flake and McCain Tuesday whether they support the amendment, which is expected to be considered on the floor of the Senate this week, neither one committed.

“Senator Flake is reviewing the amendment,” his spokeswoman, Genevieve Rozansky, told me by email.

McCain spokesman Brian Rogers was more expansive: “As it is forced to consider how to reduce costs in this new budgetary environment, Senator McCain has asked the Air Force to justify any force structure changes it may make and explain the impacts of divesting certain systems. He is committed to making sure that, whatever force structure changes the Air Force makes, the Defense Department remains capable of providing our soldiers and Marines in combat with the ‘close air support’ they need, which is the capability that the A-10 has so ably provided to date.”

Of course, that’s exactly what Ayotte’s amendment was intended to do. It requires the Air Force secretary to certify that the F-35A is fully operational and able to perform close air support, the A-10’s primary role, before allowing the Air Force to retire the A-10.

So what gives?

In all likelihood, it’s all about the F-35. The F-35 Joint Strike Fighter is the most expensive weapons system in U.S. military history, and it’s intended to replace the A-10 and other aircraft.

Phoenix scored a coup last summer when the Air Force chose to base three squadrons of F-35s at Luke Air Force Base. It meant a big investment in re-fitting the base and, perhaps more importantly, an indefinite lease on life for Luke.

Some in Tucson have campaigned vigorously for the F-35 to be based at D-M, but others say it is simply too loud to operate in this urban environment.

For members of Congress such as Flake and McCain, who hope to convince Air Force officials to send more F-35 missions to Arizona, it could be risky to criticize the program by publicly proclaiming it’s not up to replacing the old, dependable A-10.

In Congress, “that logic is pervasive,” said Winslow Wheeler, the head of the Straus Military Reform Project, part of the Washington, D.C.-based Project on Government Oversight. “You don’t want to piss them off, or they’ll feel less enthusiastic about porking up your state.”

Coincidentally, Wheeler’s group is hosting a conference on threats to the A-10 this weekend in Washington, D.C.

The problem facing those who support the A-10 is that the Air Force has a limited budget and the sequester is forcing further cuts.

“The Air Force only has budget for so many airplanes, and its top priority is the F-35,” Wheeler said. “It’s happy to push other airplanes out of the force structure to cough up money to help the F-35.”

So for politicians like Flake and McCain, there are risks in offending the Air Force, and there are also broader political considerations. Why would they stick their necks out for an aging aircraft based in the Tucson area, population 1 million, when they can prioritize a new aircraft system based in the Phoenix area, population 4 million?

Of course the calculations aren’t strictly political. Ayotte, whose husband was an A-10 pilot, doesn’t represent areas where the A-10 is based. She’s making her arguments on the basis that the A-10 is simply a better vehicle for close air support than is otherwise available.

Our arguments in Tucson include that, but also are more provincial. Davis-Monthan has been an anchor of our local economy for decades, and the A-10 is its central function for now. D-M was Southern Arizona’s third-largest employer at the end of 2012, with about 9,100 full-time-employee equivalents, the Star 200 reported.

So, senators, we’re fighting a rear-guard action in Tucson, as the A-10 lives out its useful life, which could go on for a decade or more. Excuse us if we ask you to join the fight.

Contact columnist Tim Steller at tsteller@azstarnet.com or 807-7789. On Twitter: @senyorreporter