On the afternoon of Aug. 25, Republican senators and House members, packed into a room in the Capitol's East Front so crowded that many stood and some sat on each other's laps, felt for the first time that Ronald Reagan will lose the election unless things change quickly.

The congressional "surrogates" who have agreed to travel the country on behalf of the Reagan-Bush ticket were summoned for a campaign briefing. It turned out to be more of what the congressmen described as a "fiasco" or a "disaster." Rep. Jack Kemp of New York, the surrogate in biggest demand, walked out in disgust.

To the lawmakers, the Reagan staff showed itself to be disorganized, pompous and unprepared for the heroic task of defeating a determine incumbent president. By dressing down an important Reagan supporter, William Casey also raised doubts about whether he is equipped to fill the campaign manager's post he has held since Feb. 26.

This is not merely the traditionally foul temper of Capitol Hill. The same things are being said by worried Reagan insiders. Nor is there much time to lose. One Reagan strategist expects the next crop of national polls will show a stunning Reagan decline. As panic sets in, Reagan will have to get orgainized.

Reagan's backers on Capitol Hill did not really understand the disarray in the Reagan campaign until that Aug. 25 briefing. "I thought it was just the usual liberal media gang trying to zing Reagan," one southern lawmaker confided to us. "Boy, was I wrong. The press aren't telling half of it."

The sloppy physical arrangements were the tip-off. Room EF100 was not nearly big enough to accomodate the surrogates and their aides. To make matters worse, Reagan headquarters sent a platoon-size briefing team ("It seemed like a hundred of them, but there must have been around 20," one congressman remembered).

Casey and Dr. Richard Wirthlin, Reagan's research chief, were present. But the briefing was done by two of Reagan's young right-wing staffers, Rich Williamson (who began the campaign of '80 managing Rep. Philip Crane's ill-fated presidential bid) and Terry Dolan. The surrogates had expected to be told exactly how they would fit into the Reagan campaign. They got nothing of the sort.

Instead, using flip-charts, Williamson and Dolan dealt with "peripheral" issues, to use the word invoked most often by the listeners. Much time was spent describing the "vindictive" personality of Jimmy Carter, documenting how he was so terribly nasty to the late Hubert Humphrey. It was at this point the Kemp walked out.

Rep. Bob Bauman of Maryland, a rising conservative force in the House, took the floor (as we reported earlier) to plead for congressional consultation on decisions such as George Bush's dismal trip to China. What we did not report was Casey's reaction. When the meeting broke up, Casey sought out Bauman to upbraid him for speaking up.

"You're perfect, aren't you?" the tall, lanky Casey, a 67-year-old New York lawyer, snarled to the short, stocky 43-year-old congressman. "I suppose you never made a mistake." He then committed the unforgivable sin for a campaign manager, putting the blame on the candidates, Reagan and Bush, for the China trip. "I didn't make that decision," said Casey. "They did."

Casey and Co. neither want nor accept advice from the best minds on Capitol Hill. Rep. Thomas Evans of Delaware, a former Republican national co-chairman who is supposed to be a senior Reagan adviser, has found himself ignored. Innovative freshman Rep. Newt Gingrich has been trying to interest the Reagan staff in his insightful views into his fellow Georgian in the White House, to no avail so far.

The discussion between Reagan and the Hill on the announced Sept. 15 mass pledge by Republican congressmen, standing on the Capitol steps, to cut taxes and rebuild the economy have been a shambles so far. Kemp, who initiated the idea years ago, is so disgusted he might not even participate.

A sign of hope emerged last week when the Reagan staff finally agreed to Rep. Evans' pleas for an advance look at the candidate's speeches. Evans received the Labor Day speech first draft, which like Reagan's other first drafts lately turned out to be woefully inadequate.

To many Reagan staffers who, like Alfred E. Neuman, take a "What, me worry?" view, the flak from Capitol Hill is a nuisance that should be disregarded. But a campaign operation containing few veterans of elective politics and lacking fanatically ruthless lieutenants of the type President Carter enjoys might well look to those congressmen who sniffed the fumes of defeat in Room Ef100.