The key symbol of the election debate over national defense may not be an MX missile or B-1 bomber. Instead, it is likely to be a shiny new Army helicopter with a hole in its control panel where a radio should be.
At least that is the picture of US military capabilities Democrats will paint as they try to show that the Reagan administration has overemphasized weapons procurement at the expense of fighting capability.
The watchwords in the Democratic platform, in presidential candidate Walter F. Mondale's rhetoric, and in the assertions of former senior defense officials who served in previous administrations are ''readiness'' and ''sustainability.''
Administration officials and the Joint Chiefs of Staff contend that as measured in quality of new recruits, levels of training, new weapons systems, and support equipment, the armed services ''are significantly more capable today than they were in 1980.''
Responding Monday to recent criticisms about military readiness, Gen. John Vessey, chairman of the Joint Chiefs of Staff, said that the US Air Force is able to provide 62 percent more combat sorties in support of NATO forces, that the US is now able to transport 25 percent more tonnage to Europe by air than four years ago, and that the Pentagon has spent more in the past three years improving sea lift than in all the years since World War II.
Yet, according to the Defense Department's own reckoning, the actual combat ability of US forces is less than hoped for.
A recent Pentagon report on war-fighting capability said the Navy's supply of the modern air-to-air missiles necessary to protect aircraft carrier battle groups remains ''far short'' of its objectives. These carrier battle groups are the showpieces of the administration's 600-ship fleet.
While the number of annual training days per Army battalion has remained roughly steady, the figure has dropped slightly for the Marine Corps since 1981. Air Force and Navy pilots are now getting about an hour more per month in flight time. But monthly flying hours for Army pilots have decreased every year since 1980. The Pentagon attributes this decline to insufficient money for spare parts , a condition it says came to light in 1982.
Investigators with the House Appropriations Committee found that Army aviation units often had to ''cannibalize'' some new Blackhawk helicopters for parts in order to keep the rest flying. During one six-month period, the 101st Airborne Division had to do this 146 times, the congressional probers reported recently.
''While it is incongruous that new systems lack sustainment, it is the general rule rather than the exception,'' the congressional investigation showed of the Army's equipment readiness.
Defense Secretary Caspar W. Weinberger said that the House staff report was filled with ''misstatements and errors in interpretation,'' which he termed serious and potentially dangerous.
There is, of course, a political cast to all of this. Democrats recall that similar charges about military readiness were made by candidate Ronald Reagan four years ago.
The House Appropriations subcommittee on defense, which reported the readiness shortfalls (through fiscal year 1983), is controlled by Democrats. And members of Congress themselves have in some cases slowed attempts to rebuild war-fighting capabilities.
During the past three years, for example, lawmakers have cut requested increases for Marine Corps war reserve ground munitions by 12 percent, 30 percent, and 14 percent, respectively.
Under the Reagan administration, funding for new weapons procurement has been accelerating at about twice the rate of that for operations and maintenance.
''Democrats for Defense,'' a loose coalition of former Pentagon officials, charges that ''this blind stress on big-ticket procurement is destroying the balanced defense posture this country needs, as if a family bought so many sports cars (that) it could not afford an iron or a toaster.''
The 1984 Democratic Party platform promises to ''modernize our conventional forces by balancing new equipment purchases with adequate resources spent on training, fuel, ammunition, maintenance, spare parts, and airlift and sealift....''
Civilians now in charge of the Defense Department point out that funding for war reserves did not keep pace with force modernization (new weapons) during the latter years of the Carter administration. And they point out that it takes about two years for increases in spare parts and ammunition budgets to be felt in the field.
For example, funding hikes for war reserve munitions in the 1982-84 defense budgets are expected to show up as 1983-86 ammunition increases of 14 percent for the Army, 24 percent for the Marine Corps, 58 percent for the Navy, and 62 percent for the Air Force, according to Pentagon reckoning.