Washington — If the shock of President Reagan's first economic defeat in Congress didn't register on the Richter scale, it still set Capitol Hill vibrating.
For only the second time since he took office, Mr. Reagan watched both houses muster the two-thirds majority required to override his veto. And since the earlier override was on a minor copyright bill, the action last week on a $14 billion spending measure looms even bigger. It tarnishes the President's image as ''unbeatable'' in spending fights.
Senate majority leader Howard H. Baker Jr. (R) of Tennessee went even further before the vote, calling the issue one of ''who's running the ship.'' Fellow Republican Sen. Bob Dole of Kansas warned his colleagues that ''this would at least cause a pause, and people would wonder if the President has weakened.'' But after the vote, Senator Dole, fully conceding his flip-flop, said, ''It probably won't have any big impact.''
That quick reassessment will probably prove true as the 97th Congress rushes to complete its work this month.
The dispute over the $14 billion spending measure, which will close out the current fiscal year, centered on only a few hundred million dollars - a small amount by federal standards.
''This thing is a little tadpole, and we've got a whole lot of big fish to harpoon,'' says a senior GOP Senate staffer.
The big fish are the spending bills for fiscal 1983's budget of $769.8 billion, which takes effect Oct. 1. Congress goes to work on those bills with a powerful veto threat still hanging over it. And the next Reagan veto almost certainly will be sustained.
Even as Republican senators voted to override, they signaled that next time they'll be on the President's side.
''I'm sure that there will be bills in the future that he'll be well advised to veto, and I will support him,'' Sen. Mark Andrews (R) of North Dakota said after helping to lead the override fight on the $14 billion bill. ''This is not one of those excessive cases,'' he added.
In the eyes of many in Congress, the real issue in the veto dispute was not money. It was authority.
The $14 billion spending bill, which now becomes law without the President's signature, holds spending well below budget and even lower than the amount the Reagan administration had requested.
The President and some of his advisers objected not so much to the totals, but to the way Congress would spend the money.
Reagan had asked for more for the military and less for domestic programs than Congress gave, so he issued his veto. After six successful vetoes, he had reason to believe this one would stick as well.
But this veto of a bill that did not technically ''bust the budget'' stepped over the boundary of congressional authority. It looked dangerously close to an attempt to gain ''line item veto'' for Reagan over every program Congress funds.
''I didn't come down here to be a rubber stamp for any president,'' said Senator Andrews, echoing his colleagues who saw the veto as a threat to the legislature. ''Congress has to be a coequal branch,'' he said.
That message came resoundingly from the House, which voted almost 3 to 1 to override the veto, and from the Republican-controlled Senate, where the override had votes to spare, despite a final 60-to-30 tally. Senate majority whip Ted Stevens (R) and Sen. Frank H. Murkowski (R), both of Alaska, switched their votes to save the President from a worse defeat.
Although it was the most spectacular loss for Reagan, the veto override is not his only major reversal. In May of 1981, Reagan committed a political blunder by proposing a major social security overhaul. It bombed on Capitol Hill , and the proposal was withdrawn.
Reagan met with almost as much outrage when he announced that racially segregated private schools could have tax exempt status. The proposal was soon dropped.
However, in economic matters, so successful has the President been that he convinced Congress to pass a historymaking tax hike just months before elections.
The veto setback is far less serious than a loss on the tax bill would have been. It serves as a ''don't tread on me'' signal from Congress, but it does not mean an end to Reagan's dominance in cutting government spending.