DES MOINES, JAN. 20 -- Senate Minority Leader Robert J. Dole (R-Kan.) launched an unusual daylong attack on rival Rep. Jack Kemp (R-N.Y.) today, saying Kemp is trying to "prey on the elderly" with a political mailing here criticizing Dole's vote to curb Social Security cost-of-living increases in 1985.

Dole repeatedly held up for audiences today a brown envelope marked "Important Social Security Information Enclosed." The envelope includes a Kemp flier attacking Dole and Bush for the 1985 Social Security vote and includes a newspaper clipping describing Bush's role as a tie-breaker.

Kemp aides said the mailing is being sent this week to 120,000 Iowa voters over 50 years old. He has also raised the issue in television ads here.

Dole, who is leading in Iowa and enjoys strong support among older voters, usually ignores Kemp, with whom he has had long-running disagreements on economic policy. But in a series of campaign speeches today, Dole aimed angry volleys of criticism at the New York congressman.

"If Jack Kemp had his way, the envelope would have been empty, there wouldn't have been any checks for Social Security," Dole told the Ames Chamber of Commerce. He repeatedly described Kemp as the "free-lunch candidate."

Later, in a speech to insurance company workers here, Dole added, "I want the people of Iowa to know Jack Kemp voted against the Social Security rescue package" in 1983. "You can't have it both ways.

You can't be the free-lunch candidate."

In a telephone interview, Kemp responded that "Bob Dole is preying on the elderly because he wants to freeze Social Security benefits." Kemp said there was "nothing free lunch" about the economic growth that followed the 1981 tax cuts. "I am the growth candidate and he is the bitter-medicine candidate," Kemp said of Dole. The congressman said he voted against the 1983 Social Security package because it included a tax increase.

In stump speeches, Dole has described at length his role in the 1983 Social Security rescue package. The package increased payroll taxes and reduced some benefits in an effort to restore the huge pension system to financial stability; Social Security has also been aided by the improved economy in recent years.

Dole acknowledged casting the controversial 1985 vote described in the Kemp flier, and defended it as necessary to reduce the deficit. He has been making deficit reduction a centerpiece of his campaign, saying he was more effective than Kemp or Bush at curbing the red ink in recent years.

Dole was a leader in the 1985 deficit-reduction effort that included a freeze, approved by the Senate, on cost-of-living adjustments for several programs, including Social Security. President Reagan abandoned the effort, however,

and it was never enacted by the House.

The 50-to-49 Senate vote approving the freeze, in which Bush broke the tie and thus has been drawn into the issue, has been a political hot potato for Dole for some time. Democrats brought it up in the final days of the 1986 Senate campaigns, and political analysts said the issue played a role in the defeat of three Republicans -- James Abdnor of South Dakota, Jeremiah Denton of Alabama and Mack Mattingly of Georgia -- in a year in which the GOP lost control of the Senate.

Dole, holding up the brown envelope for reporters and television cameras here, said, "It doesn't have his name on the outside, doesn't say Kemp for president." Dole added, "That's getting marginal." The mailing does identify Kemp as its source on the inside, and Kemp denied it was "negative" advertising "unless Bob Dole thinks his vote was negative."

Dole quoted Kemp as saying he was trying to protect Social Security benefits. "Sure he is," the senator said sarcastically. "Maybe you get a few votes from senior citizens who are scared and they open up a brown envelope and it's unmarked and they say, 'Jiminy, I didn't know Bob Dole was going to hurt me.' I'm not going to hurt anybody . . . . "