A self-styled Reform Caucus has been formed in Congress that is staunchly pro-Pentagon but nevertheless has warned the Reagan administration it may have to chop back the Pentagon budget dramatically, beginning perhaps this fall.

The group contains conservatives and liberals of both parties. Its founders are Rep. G. William Whitehurst (R-Va.) and Sen. Gary Hart (D-Colo.). It makes the strongest expression to date of a view held by a growing number of Pentagon defenders in Congress:

That unless some convincing show of economy is made, there will soon be a backlash in Congress against the defense budget increases President Reagan has proposed while domestic programs are being slashed.

The new caucus, which already has 16 members although it is not recruiting, has met with Defense Secretary Caspar W. Weinberger and his deputy, Frank C. Carlucci.

Congress is well on its way to approval of a record $222 billion defense budget for the next fiscal year. For 1983 and thereafter, Reagan has proposed increasing the total by 7 per cent a year after allowing for inflation. Now defense officials say even this may not be enough to buy all the weapons Reagan wants, but budget officials say it may be more than the government can afford.

"We're afraid," Whitehurst said in an interview yesterday, "that when the full impact of these domestic cuts is felt, when people say, 'Doggone it, this really smarts!' they're going to say, 'Why can't some big cuts be made here?' Then there will be a big hue and cry to go after the defense budget."

Whitehurst, second-ranking Republican on the House Armed Services Committee, said the pro-defense coalition which pushed Reagan's record high military budgets through Congress this year could be cut to ribbons in 1982 by resentment against cuts in such areas as education, Social Security and health care.

At the unannounced breakfast with Weinberger and Carlucci at the Capitol June 16, caucus members suggested very specific budget cuts.

Cancellation of the Navy's F18 fighter program and building a mix of expensive nuclear and less costly diesel attack submarines, rather than just the nuclear ones, were among the suggestions, Whitehurst said. This is asking the new team at the Pentagon to take on some of the most entrenched bureaucracies in the military and Congress. But Whitehurst and his allies insist the time has come for the confrontation.

"The idea of spending $40 billion on the F18, which was supposed to be the low end of the high-low mix of weaponry and is now the high of the high-high, is just choking us and splitting the Congress," Whitehurst contended.

Similarly, he argued, two nuclear attack subs and three diesel ones could be bought with the same $1.5 billion that the Navy intends to spend on just three nuclear boats. The coalition will force the Pentagon to consider buying a mix of the Cadillacs and Chevrolets of weaponry in the belief that quantity as well as quality decides battles.

At the breakfast, Weinberger pledged to set up "a B team" of specialists to take a fresh look at such alternatives before committing much more of the $1.5 trillion Reagan has earmarked for defense for the next five years.

The caucus, which is still being quietly organized, will operate outside the regular congressional committee structure with its own staff, Whitehurst said. How it will be financed has not been worked out yet, he added. The idea of a free-swinging, cage-rattling group to force the Pentagon to examine alternatives to long-standing and conventional wisdom has so far prompted these lawmakers to sign on, Whitehurst said:

Sens. William S. Cohen (R-Maine), Hart, Sam Nunn (D-Ga.), John W. Warner (R-Va.), all of the Armed Services Committee, and Arlen Specter (R-Pa.); Reps. Charles F. Dougherty (R-Pa.), David F. Emery (R-Maine), Thomas M. Foglietta (D-Pa.), Paul S. Trible Jr. (R-Va.), all of the Armed Services Committee; Norman D. Dicks (D-Wash.) and Jack Edwards (R-Ala.) of the House Appropriations subcommittee on defense; Douglas K. Bereuter (R-Neb.), Dick Cheney (R-Wyo.), Newt Gingrich (R-Ga.) and Marge Roukema (R-N.J.).

"I feel I'm about to climb Mount Everest," Whitehurst said of the group's effort to force new thinking on the Pentagon. "But if we don't do it, and something happens down the pike, we'll be sorry we didn't try."