Former president Gerald R. Ford, who this weekend virtually invited fellow Republicans to demand that he enter the presidential race, would face some formidable obstacles should he decide to challenge again his old adversary, Ronald Reagan.

Ford issued the invitation in an interview published yesterday in the New York Times in which he said Reagan, as a "very conservative Republican," would face "an impossible situation" in the November general election and would be defeated by the Democratic nominee.

If there was an "honest-to-goodness bona fide urging by a broad-based group in my party, I would respond." Ford was quoted as saying about suggestions that he enter the GOP presidential race as an alternative to the front-running Reagan.

Reagan, back in New England seeking to build on his New Hampshire triumph, took the Ford statements with aplomb. "Well, We'd all like to see him pack his long johns and come out here on the campaign trail with us," he said.

Bob Barrett, Ford's executive assistant, yesterday portrayed the former president as a man grown weary of the increasing number of private urgings that he enter the race. So he decided to force the issue publicly by asserting that Reagan is a sure loser in November.

"The guy's a little bit ticked off," said Barrett in a telephone interview from Ford's home near Palm Springs, Calif. "He's tired of people whispering in his ear. His attitude is, get out and shout it and I'll listen. That's the sum and substance of it."

Barrett, known in political circles for his behind the scenes promotion of a Ford candidacy, said the next seven to 10 days will be crucial to Ford's decision as he evaluates Republican reaction to his invitation.

Ford is scheduled to be in Washington March 11-13, at which time he is certain to be pressed for a definite answer on his political plans.

The timing of Ford's statement suggests that he recognizes he is rapidly approaching a point of no return in deciding whether to try to head off Reagan, whose landslide victory in last week's New Hampshire primary placed him in a commanding early position in the Republican presidential field.

One of the main obstacles Ford would face is the imperatives of the political calender. Republicans will send 1,194 delegates to the national convention in Detroit July 14-17, withthe votes of 998 delgates needed to win nomination.

But even as Ford was making his strongest statement to date about a possible candidacy, the filing deadlines for 17 state primaries, which will elect 774 delegates, had passed.

But by the end of this week, filing deadlines in four states with 134 delegates will have passed. And by March 21, three days after the important Illinois primary, filing deadlines in 28 states with 1,339 delegates will have passed.

Beyond these barriers, and the financial and organizational problems that beset any late-starting presidential campaign, Ford would risk incurring the anomosity of other Republicans who have been battling Reagan for months.

This was evidenced by the reaction of George Bush, whose campaign organization includes a number of Ford loyalists who waited to join Bush until they were sure the former president would not be a candidate this year.

"If he wants the nomination, he's going to have to compete like everyone else -- and it's tough out there," Bush said in Hyannis, Mass., where he was campaigning yesterday. "No one is going to hand him anything."

An even sharper comment came from David Sparks, Bush's deputy national campaign manager, who in 1976 ran Ford's primary campaign in Massachusetts.

Ford, Sparks said, "just reacts when a microphone is shoved in his face at whatever celebrity gold tournament . . . Maybe 18 holes a day isn't enough for him."

Ford's weekend statement appeared to catch many political associates by surprise. Robert Teeter, Ford's pollster in 1976 who is now working for the Bush campaign, said he knew of no change in Ford's plans since last fall, when, shortly after meeting with Teeter and other advisers, the former president eliminated himself as an active candidate and urged his supporters "To jump into the fray in behalf of the candidate of their choice."

Another Ford Associate said talk of a Ford candidacy, particularly among GOP members of Congress, had picked up noticeably since Reagan's big win in New Hampshire. But he said he was unaware of any planning for Ford to enter the race.

None of the Ford associates discounted the possibility that the former president could enter the race late and still win the nomination. But they conceded the difficulty he would face with a late-starting campaign, a message they had pressed on Ford during the meeting last fall.

Barrett and others also suggested Ford will face a de facto deadline for his decision around March 20 or 21. On those two day, four states with 357 delegates -- Maryland, Ohio, Ford's native Michigan and Reagan's home state of California -- have primary filing deadlines.