Coniston, Unions to Try UAL Buyout : Proxy fight: Joint effort hopes to buy airline’s parent for more than $3.8 billion.

NEW YORK —  Three unions and one major shareholder announced a joint effort today to buy United Airlines’ parent corporation for more than $3.8 billion.

The offer, based on a plan to install new directors in a shareholder proxy fight, shattered a lull in the prolonged takeover maneuvering for UAL Corp. It included a commitment by the unions to cut labor costs and not to strike.

The $3.8 billion is about 40% less than a debt-financed offer that collapsed five months ago, touching off a major tumble in the stock market and raising the first big doubts about the future of the corporate takeover craze of the 1980s.

Coniston Partners, a New York takeover firm that has invested in UAL and has long demanded changes to boost the stock price, disclosed details of the joint plan with the pilots, machinists and flight attendants unions of the Chicago-based airline, the second largest in the country behind American Airlines.

It said they plan to lobby for shareholder proxy votes in favor of electing a new slate of directors at UAL’s April 26 meeting, and the directors will try to force the company to agree to the takeover proposal.

Stockholders would receive $150 in cash and $25 in securities, as well as non-voting common stock amounting to 25% of the total in the company, the proposal said. The unions would own the remaining 75%.

Based on UAL’s 21.8 million shares outstanding, the offer would be worth about $3.8 billion.

UAL was trading at about $142.50 a share on the New York Stock Exchange before the announcement, then shot up to $151. The shares have traded as low as $121.

Stephen M. Wolf, chairman of the company, said in a statement that the board of directors was prepared to consider the Coniston-union proposal and would meet promptly to discuss it as well as other unspecified alternatives.

It was unclear how Coniston and the unions intended to finance the takeover. But their announcement said new labor agreements would be negotiated that would include no-strike commitments and “result in substantial labor cost savings.”

The collapse of a debt-financed $300-a-share offer for UAL on Oct. 13 is considered one of the major events in the corporate takeover era because it instilled deep skepticism about whether such deals would continue. The Dow Jones industrial average plunged 190 points that day.

The subsequent collapse of the junk-bond market practically stopped all takeover deals because it eliminated the major source of financing. Takeovers for airlines in particular are now considered riskier because of uncertainty about the economy and the overall health of the airline industry.