Did President Reagan let presidential campaign politics determine his decision this week to veto the controversial $299.5 billion defense authorization bill?

That's what congressional Democrats are insisting. And certainly the veto sharpens the differences between Republicans and Democrats on defense issues. But even if this were not an election year, the veto would have been justified.After being adjusted for inflation, the $299.5 billion figure represents a decline in U.S. defense spending - the fourth such annual decline in a row. Because of the compromise to which he agreed at last fall's budget summit with Congress, President Reagan is locked into that figure. But national security demands that he not accept some of the dangerous tinkering Congress has done within that total.

Even the Carter administration recognized the need for a land-mobile MX missile system to counter similar Soviet missiles. But Congress has gutted the Pentagon's MX mobility program, reducing its 1989 funding from $793 million to $250 million. Russia, incidentally, has come up with two new classes of land-mobile ICBMs in recent years; the U.S. has yet to deploy a single such weapon.

At a time of massive Soviet strides in submarine warfare capability, Congress also seeks to retire two serviceable Poseidon-missile submarines.

Then there's the lawmakers' attempt to cut the heart out of strategic defense research, reducing funds for space-based interceptors from $330 million to $80 million. This cut, too, comes at a time when Moscow has embarked on a crash program to deploy an anti-missile defense system.

By vetoing this bill, President Reagan has in effect invited both presidential candidates to spend more time explaining how they would provide for America's defense. Call that election-year politics if you will. But it's also an explanation that the voters deserve to get.