In the few minutes before he went on television Friday night, the president was complaining how the lights hurt his eyes. They were the brightest he'd ever experienced, he said to the technicians around him. Outside, the cameras caught a wintry scene -- snow swirling around a branch standing isolated before the White House in the background -- as the setting for the somber presidential address.

Both glimpses captured that moment, for the president and for the nation, Jimmy Carter now faces the critical juncture in his presidency, the greatest heat he's experienced in nearly three years in the White House. The nation enters a new period of tensin and danger. It's going to be a cold winter in Washington -- and the world.

The president's speech on the Soviet Union and Afghanistan came exactly 60 days after the seizing of American hostages in Iran. In that period a profound change appears to have taken place.

For the first time in at least a generation all segments of this diverse society stand united against a commonly perceived enemy: the Iranians, personified by the Hitler-like figure of evil, the ayatollah. Politically, this crisis has resulted in the most dramatic reversal of a prsident's fortunes in decades.

What's extraordinary about the sudden rise of Carter from the political depths during that two-month period was the way in which it was achieved.

The president was hardly seen or heard during that entire passage of time. He delivered no address to the nation on the crisis; his major report on where we stood and what we faced and what he felt and thought came in a news conference, the only one held throughout those weeks.He canceled a number of scheduled trips -- to Florida, to Georgia over Thanksgiving, to Canada, to Pennsylvania, to Oregon and Washington -- always, in the words of the brief White House statement explaining why, because he wished "to continue to closely monitor the situation in Iran."

Two days after the hostages had been taken, he accepted an invitation to debate his principal Democratic challenger, Edward M. Kennedy, in Iowa. Just before New Year's Eve, he withdrew from that debate. He said he was doing his best "to keep the public informed about Iran and about other important matters of concern to our country. For the time being, I can best continue these efforts if I forgo personal appearances or participation in events which are exclusively part of a partisan political campaign."

His policy of restraint and patience, aimed at first getting the hostages back unharmed, virtually barred any political criticism of U.S. policy toward Iran or the circumstances of the shah's admittance to the country. To inititate such a debate was akin to being unpatriotic.

"At this time, I am not interested in trying to resolve whether or not the shah was a good or bad leader or of the history of Iran," the president said on Dec. 7 in brief extemporaneous remarks to government workers in the State Department lobby after meeting privately with families of the hostages.

"I'm not trying to interfere in the government of Iran or the inclination of the people there, and we do not want to confuse the issue by injecting these extraneous questions or debates into the present situation. When that does happen, in my opinion, it delays the day when we will see the American hostages come home."

While the president apart, monitoring the crisis from the White House or Camp David, and his political rivals by and large were stilled the forces of emotion coalesced to propel him upward in the polls. Daily scenes of the screaming Iranian mobs shouting insults against the Americans did something beyond the ability of any president in recent years.

It gave us an enemy, a vehicle to vent so many frustrations accumulated during the Vietnam-Watergate era. And Americans did something they always have done: they rallied behind the single symbol of the national will and purpose, the president.

The president was far from being non-political, of course, any less than at his lowest point last August when he traveled down the Mississippi aboard the Delta Queen. That "vacation" trip resulted in his making some 48 speeches along the way in such politically important midwestern states as Iowa -- and strongly rejecting the criticism that he could better mind the country's problems and combat what he had described as the national malaise by working in the White House.

In the Iranian situation, he has been making daily calls to voters all over Iowa, some on telephonic hookups with groups of people reportedly, lasting from 20 to 30 minutes. "If he's got to be calling activist Democrats around the state, then he's got time to debate," Ralph Scharnau, of Dubuque told The Des Mones Register and Tribune the other day.

But another change also has been in the making. As the Iranian crisis continued without prospect of resolution, day after frustrating day, the pressure to "do somthing" intensified. Carter's policy of restraint increasingly is being criticized as a policy of impotence.

Then came the Soviet invasion of Afghanistan. Before the president spoke Friday night, some of the nation's shrewdest professional political analysts privately were saying that Carter was in peril of another precipitous drop in the volatile opinion polls. As one of them said, only hours before the speech was announced. "The air is slowly going out of his balloon."

All this hardly marks an auspicious beginning for the 1980s.

The most difficult kinds of challenges at home and abroad almost certainly lie ahead. And now, with Iran still the same, if not worse, and political conditions unraveling again internally, the president and the nation are heading in another direction amid even greater uncertainty.