Washington — Gary Hart, his ''new ideas'' campaign at an end, now faces a crossroads in his high-speed political career. One signpost points to the US Senate, where the prospects are for a difficult , costly reelection battle in the next two years.
The other sign is turned in the direction of another run for the White House in 1988 - assuming Walter Mondale and the Democrats lose this year.
Several friends and associates of the senator expect him to choose the second road. They predict he will drop out of the Senate when his current term ends in January 1987 to devote full time to the next presidential race.
''Gary has tasted it, and there is no turning back now,'' one friend from Colorado says. ''The assumption here is that the Senate no longer will be a strong attraction for him.''
Within Hart's circle of friends, however, the wisdom of another run for the White House is already being quietly debated.
One person close to the senator says candidly:
''I think he should stay in the Senate. That's where he can make his greatest contribution. And it is the environment in which he is happiest.''
In private discussions, there are strong arguments urging Hart in each direction.
Those who want him to drop out of the Senate and look toward the 1988 presidential contest often point first to Hart's huge debt left over from this year's race.
He owes an estimated $3.5 million to $4 million. He hopes to get help from Walter Mondale and the Democratic Party to pay it off. Running again for the Senate would require him to raise an additional $2 million more - no small amount in Republican-leaning Colorado.
They also observe that four years ago, Hart won reelection to his Senate seat by a narrow 1 percent margin against only moderate Republican competition. If a strong GOP foe takes the field against him this time, he could face an embarrassing defeat.
These friends add one more argument, an agrument based on recent political history.
Not a single one of the last three elected presidents - Ronald Reagan, Jimmy Carter, or Richard Nixon - was holding political office at the time of his election. Mr. Carter, for example, had nearly two years after his term as governor of Georgia to devote to full-time campaigning.
Hart's friends consider his Senate obligations a drawback if he is to make an all-out effort in 1988. They also observe that he really doesn't need the Senate stage any more to further his career: He now has a national reputation and thousands of devoted followers from New Hampshire to Iowa to California.
There is, however, a second view among some of Hart's friends. Their view was epitomized at the moment Hart stood before the Democratic convention on Wednesday night, accepting the cheers of the delegates.
It was a golden moment - the capstone of his presidential campaign, the reward for months of hard work, sleepless nights, hectic weekends, endless travel. Yet at that moment, Hart appeared uncomfortable. He was tight-lipped, cool, unresponsive. Several of his aides noticed it, and were disappointed.
One friend who was watching him that night strongly feels that Hart should forgo any further efforts for the White House. Hart, this friend says, really isn't ready for it. His roots aren't firmly enough planted. He still has family problems. And he personally hasn't a good enough grip on who he is, and what he is all about, the friend says.
Hart's proper place, the friend argues, is in the Senate. There he can continue to mature. He can promote his ideas on the economy, the Pentagon, and foreign affairs. He can get the time he needs to work out his personal problems.
Even so, the temptation for Hart will be tremendous if Mr. Mondale loses this year. Many political observers then would automatically consider Hart to be the front-runner for 1988.
Across the country, many Democratic activists would look to him for guidance, and for a signal that he was ready to make the run again for the White House. Could he just pass all that by for a quiet committee assignment on Capitol Hill?
Hart's friends were not the only Democrats thinking about 1988 at the San Francisco convention. The race four years from now was in just about everyone's thoughts after the emotional speech Monday night by New York Gov. Mario M. Cuomo.
In another year and another kind of convention - before primaries replaced smoke-filled back rooms - Governor Cuomo would have been just the kind of crowd-pleaser who might have been grabbed by party leaders and put at the top of the ticket.
Instead, this time many of the delegates talked of him as a serious challenger to Hart for the front-runner spot in 1988.
Many delegates clearly are excited by the party's prospects in '88. Besides Hart and Cuomo, a whole list of possible contenders was being mentioned. Among them:
Jesse Jackson. His fiery call for party unity won the hearts of hundreds of black delegates and drew new support for him from some other party faithful.
Edward M. Kennedy. The Massachusetts senator is believed to have held back for family reasons this year. In 1988, the track could be clear for another try. His final day speech was a rouser to those in the steamy Moscone convention hall.
Geraldine A. Ferraro. If she conducts herself well this year, Representative Ferraro has to be considered a strong possibility in 1988.
John Glenn. The Ohio senator was a surprisingly weak candidate this year, but it would be no surprise if he tried once again.
Joseph Biden, Bill Bradley, Sam Nunn. All three senators are mentioned as possible candidates in '88.
In addition, there are a number of young, upcoming governors, such as Jay Rockefeller of West Virginia who show an interest.
One encouraging aspect for all of these potential candidates was the upbeat convention in San Francisco. There were problems, of course. Many black delegates, for example, left the city feeling ignored by Mondale.
Yet as the delegates waved thousands of tiny American flags and actress Jennifer Holliday led the delegates in singing ''The Battle Hymn of the Republic'' on closing day, Democrats were as united as they have been in recent memory.
The contrasts with 1968 (riots in Chicago), 1972 (splits over Vietnam), and 1980 (the Kennedy-Carter clash) were dramatic.
Mondale's closing speech - ''we are wiser, stronger'' now, he said - also helped to heal divisions.
Many political analysts still expect the Democrats to lose this year. But the contest now looks closer, and the party's prospects better, than only one week ago.