Securities industry leaders today predicted that tax legislation cutting the holding period for long-term capital gains from one year to six months has a 40 percent chance of enactment before Congress adjourns this year.

The call for the tax law changes has long been a major priority of the Securities Industry Association, which is meeting here this week. A congressional conference committee dropped the tax provision in August during debate over the tax bill.

According to George L. Ball, chairman of Prudential-Bache Securities Inc. and chairman of the SIA's tax policy committee, the measure is likely to be attached to Senate legislation dealing with utility rates in California.

Both Ball and Robert E. Linton, chairman of Drexel Burnham Lambert Inc. who is to be installed as the new SIA chairman, agreed that the measure has a 40 percent chance of passage before the new Congress convenes next month and a 60 percent chance of enactment next year. Ball said the biggest obstacle to passage of the bill is the prospect in the Senate of several unnamed members putting a hold on the entire piece of legislation.

Among the obstacles to the change had been the opposition of Rep. Dan Rostenkowski (D-Ill.), chairman of the Ways and Means Committee. But Edgar D. Jannotta, the 1982 SIA chairman, told the association today that under the "right circumstances," Rostenkowski has told him he would support the heavily lobbied SIA proposal.

"When these right circumstances will occur, I don't know," Jannotta said. "But I do feel that for the first time we have all the parties, White House, Senate and House, fundamentally supportive. And sometime between now and the next Ice Age, the reduction should occur."

The capital gains issue is among the major topics of discussion on the SIA agenda this week, though the meeting has only a limited business schedule permitting a full schedule of golf, tennis and swimming for the 980 registered industry participants.

The other major topic occupying SIA members here is the industry's fierce opposition to a suggestion floated by New York City Mayor Ed Koch's administration that the state government might reimpose a stock transfer tax dropped last year.

Industry leaders, including SIA officials and John P. Phelan Jr., president of the New York Stock Exchange, vowed to fight the proposal, which Koch suggested recently deserved state government attention. In fact, the threat of an exodus from Wall Street by securities firms whose presence there had become synonymous with the lower Manhattan street, was raised in an effort to block consideration of such a move.

"With the securities industry increasingly automated, it becomes easier to move operations out of New York," said Edward I. O'Brien, president of the SIA.

"Reimposition of the transfer tax--which on many transactions would be greater than commissions--could provide the impetus for firms to leave New York. The very real possibility of such an exodus was the reason the legislature decided" to stop collecting the tax.

The two issues were the most pressing concerns before the group. Otherwise, most executives and brokers seemed content to bask in both the Florida sunshine and the ballooning profits of the industry, which has been the major beneficiary of the fall's surging stock market.