THIS is the moment of truth for Mikhail Gorbachev. If he plays his hand very deftly, he can probably go to a summit meeting with President Reagan by year's end.
That was the underlying message in President Reagan's address to the United Nations Monday.
There is a good chance that at that summit, the Soviet Union and the United States could agree on some significant reductions in nuclear weapons.
That would ease tensions between the two countries and relieve a lot of pressure at home on Mr. Gorbachev, who is promising Soviet citizens a better life but presiding over a mismanaged Marxist economy.
The moment to achieve all this is now.
If General Secretary Gorbachev lets the opportunity slip away, he must face another two years with an antagonistic Reagan, who is not likely to make the effort again.
And whether Mr. Reagan is succeeded by a Republican or Democrat, there will be another two years before his successor gets his feet on the ground firmly enough to negotiate a fresh deal with the Soviets.
Although Gorbachev is a young man, four years of procrastination and non-progress with the United States is something he must ponder seriously.
In his speech, Reagan projected a statesmanlike tone. There was talk of forces ``greater and higher than ourselves.'' There was the desire to ``beat swords into plowshares.''
There were the signals to the Soviets that he is taking them seriously. Last year's Geneva talks had been ``frank and productive.'' Latest overtures suggested a ``serious effort'' by the Soviets.
All this, said Reagan, gives him ``hope . . . that the ice could break.'' He was, he said, giving ``serious and careful consideration'' to Gorbachev's most recent letter on arms control.
But the price of progress is clearly the freeing of Nicholas Daniloff, the American reporter trapped by the Soviets and held on what Reagan called ``fabricated accusations and trumped-up charges.''
Reagan said the Soviets' action on Mr. Daniloff was a ``transgression against human rights.''
The message to the Soviets was clear: There are signs of movement in our relationship; we may be able to cut a deal; but nothing is going to happen until you set the Daniloff matter right.
After initially stumbling around on the Daniloff affair, the Reagan administration now seems to have gotten its priorities in order again.
They may be priorities the Soviets find it hard to understand. How can the treatment of one man hold up, perhaps jeopardize, the practice of summitry and power dealing?
That is a lesson the Soviets find hard to learn. Their atrocious record on human rights keeps tripping them up as they seek to assert their role as a world power.
If they would learn to treat people decently, many of us could forgive their national insecurities, perhaps even learn to tolerate their cockamamie economic system, discredited though it may be.
We will probably never know who miscalculated in the Soviet hierarchy when they ordered the Daniloff frame-up.
But the issue now for Gorbachev is whether that affront to human rights is to be allowed to jeopardize the better US-USSR relationship that both Mr. Gorbachev and Mr. Reagan say they are willing to work for.
The Soviets cannot be allowed to seize hostages and trade them for Soviet citizens who have transgressed American laws -- particularly espionage laws.
If Gorbachev can concede that rather basic point in the civil conduct of affairs between nations, then maybe we are going to see progress on a broader range of issues.
If he remains blinkered on this matter of principle, we can all save a lot of time by forgoing the summit.