IN A LAST-MINUTE burst of energy, the free-trade talks with Canada have suddenly leaped into motion. Today's sessions will apparently be decisive. A week ago, it looked as though both governments were going to let the whole idea collapse. That could still happen. Nothing is certain in this gigantic endeavor to tie the two countries together in a free-trade zone. But now Cabinet-level politicians on both sides have taken hold of the process, and it is moving rapidly.

It needs to move very rapidly indeed. A deadline is imposed by the American legislation establishing the fast track that the agreement must follow through Congress. After some confusion, the parliamentarians now say that it falls at midnight tonight. By that time President Reagan and Prime Minister Brian Mulroney must decide whether they have an agreement.

When the Canadian negotiator, Simon Reisman, walked out of the talks last week, that attracted attention. One of the Canadians' complaints -- and they had a real point -- was that the Americans at the table were middle-level trade specialists who lacked the standing to make political decisions. The American side was led by Peter Murphy, an assistant to U.S. Trade Representative Clayton Yeutter. He knows a great deal about trade law, but wasn't in a position to push back when senators began pushing him in behalf of local interests. Here in Washington, the big change this week has been that Secretary of the Treasury James A. Baker is now leading the American side.

In Canada as well there seems to have been a change of heart. It looked earlier as though Mr. Mulroney was preparing to abandon the free-trade zone as a proposal that turned out to be much more controversial than he expected, and an incitement to those Canadian nationalists who fear being absorbed into the United States. But over the past week, he and his Cabinet apparently decided to try again.

Political leaders have begun to think carefully about the enormous benefits of bringing the agreement off -- and, conversely, the very real costs of failure. If the agreement were to fail, one result would be a long period of very scratchy relations between the two countries, in which the protectionists on both sides, having won a great victory for themselves, would make the most of their triumph.

But a free-trade agreement would mean a great expansion of the opportunities in both countries to do business across a border that already carries more international commerce than any other in the world. Wider opportunities will benefit the most competitive and efficient industries in both countries. That's how rich nations like the United States and Canada get richer