Sen. George McGovern (D-S.D.), in a turnabout that dramatizes the pro-defense tide threatening to pull him and other liberals out of office this year, has come out for building the B1 or a bomber like it.

It was McGovern, as senator and 1972 Democratic presidential candidate, who filled the political sky with flak and tried to shoot down the B1.

But today finds of McGovern trying to climb aboard some kind of new bomber in hopes of escaping some of the political exploding all around him in South Dakota as he seeks reelection this Novermber.

This is a sample of the rhetoric being shot at McGovern: "If you and I care about our nation's security," states a letter from a retired Army general being circulated in South Dakota by the National Conservative Political Action Committee, "we must work together to defeat George McGovern this November." His votes again the B1 are among the reasons cited.

McGovern is being challenged by James Abdnor, a Republican congressman. McGovern tried to dramatize his commitment to national defense by recently visiting Ellsworth Air Force Base in his home state and issuing this statement:

"We have now reached the point where we will have to begin production plans for the B1 or some similar bomber."

He elaborates on this new position in a newsletter that will go to South Dakotans next week. His reasoning appears under the heading,"The Strategic Bomber -- A key Weapon in America's Defense Arsenal."

The senator writes that the bomber leg of the strategic nuclear triad of bombers, missiles and submarines, "must be improved" because B52 bombers are wearing out and the new MX land missile "is an unnecessary waste of taxpayers' funds" because it does not solve the vulnerability problem at a reasonable cost.

"I remain convinced," says McGovern in his news letter, that President Carter was right to Cancel the B1 bomber in 1977 and build the cruise missile instead. Improved Soviet defense, McGovern says, "will make the B1 obsolete by the end of the 1980s."

Even so, McGovern states he could support what House members call "the son of B1," a plane that would carry cruise missiles on the old B1 air-frame but have a stiff wing rather than one that swings back and forth as originally planned. This B1 derivative is called the Strategic Weapons Launcher, nicknamed SWL or Swill.

The House voted money to build the Swill in the fiscal 1981 procurement bill still awaiting action by the Senate.

Besides supporting son of B1, McGovern states in his newsletter that he would support a stretched version of anothr controversial airplane, the F111. One Air Force idea is to elongate the Fill to give it strategic range and install B1 engines in this interim bomber.

After telling constituents that he could vote for either son of B1 or son of F111, McGovern writes: "I prefer the FB111 bomber option over the B1/SWL option."

The Senate Armed Services Committee, in its version of the Pentagon procurement bill to be voted later this week, authorized $91 million to start working on a strategic bomber version of the F111.

But now what a new bomber is picking up support from such outspoken critics as McGovern, Carter and top Pentagon civilians do not want either the modified B1 as a cruise missile carrier or the stretched F111 as an interim manned bomber. They argue that Soviet defenses make both those bombers a waste of money.

Even some Air Force bomber enthusiasts fear building either a modified B1 or F111 will endanger their plans for the plane they really want. On the drawing board is a high-flying bomber that would so slow but fly so high and carry such effecitve electronic spoofing equipment that enemy radars could not detect it.