Poor Lew Allen.

Five weeks ago, he won the biggest victory of any Air Force chief of staff in 20 years. The president and secretary of defense agreed to give his service a brand new bomber. Two bombers, in fact.

But hardly had the pilots of the long-frustrated Air Force stopped toasting that decision in officer clubs than the skies over Congress turned unfriendly.

The same congressional committees that two decades ago assailed civilians in the executive branch for not building one bomber, the B70, started shooting at first of the two this new bunch wants to build: the B1.

Worse yet, the same secretary of defense who had reversed the tide against building any new bomber in this missile age, Caspar W. Weinberger, handed the congressional critics the best ammunition for shooting it down.

The B1, which will not start coming off the production line until 1986, will not be able to get through Soviet defenses beyond 1990, Weinberger told a Congress under pressure to find places to save money. It did not take long before several senators said they would be damned, by their constituents if nobody else, if they spent $28 billion for 100 bombers that would be good against Russia only for four years.

Instead of spending all that money, this new argument went, save it for the more advanced Stealth bomber the Air Force claims could penetrate Soviet defenses all through the 1990s. But, Allen frets, Stealth is like all those other bombers civilians in the White House and Pentagon have been promising the Air Force the past 20 years.

So, unless things start looking up, Allen will be where Gen. Curtis E. LeMay was during his tour as chief of staff from 1961 to 1965, that is, waiting for a paper bomber to become a metal one. Allen said if the B1, which Carter shot down in 1977, is shot down again, this time by Congress, it must be listed as a casualty of the Battle of the Budget, not a flawed concept.

This congressional fire against the B1 stems from the fact that it is such an inviting target for "a Congress which feels at the moment absolutely obliged to dig large chunks out of the budget." The old arguments which shot down the B1 the first time around, such as whether it would be a better buy than cruise missiles, are almost beside the point today because finding places to cut is the name of the game in Congress, Allen contended.

"I'm a little bit embarrassed saying this in the sense that obviously I'm speaking for congressmen who, probably if they were in the room, would deny that was their attitude. But I'm making a supposition based on a lot of conversations" with legislators.

"They would probably describe their own feelings differently," said the Air Force chief of those he sees as the biggest threat to the Air Force's fragile dream of getting a new bomber.

"My sensing of what is happening in Congress is that there is support on the merits of the case, but there is overlaying that a very deep concern about economic matters which may cause them to dip deeply into defense."