DES MOINES -- In the few days before the year's first votes for president are cast, candidates of both parties have begun zeroing in on specific issues that they think can change minds and ballots.

The most popular such issue has been Social Security, the most durable "hot button" in Senate, House and presidential races throughout the 1980s.

On the Republican side, Rep. Jack Kemp (N.Y.) has been using television advertisements, direct mail and stump speeches to accuse his principal rivals of wanting to undermine the system, raise its payroll taxes or cap its benefits.

His targets, in turn, charge him with distorting their positions and "fear-mongering" the issue in a way that they typically associate with Democratic campaigns. "I'm afraid Kemp has 'Claude Pepperized' the debate," said Dennis Rochford, Iowa campaign manager for former Delaware governor Pierre S. (Pete) du Pont IV, referring to the 87-year-old Florida congressman who champions the elderly.

On the Democratic side, Sen. Paul Simon (Ill.) this week debuted a radio commercial in which he lashes Rep. Richard A. Gephardt (Mo.) for having "led the fight in 1985 to wipe out cost-of-living increases for Social Security recipients."

The commercial also cites Gephardt's votes for a 1980 grain embargo that was unpopular here, for weapons systems such as the B1 bomber and MX missile, and for the 1981 Reagan tax cuts.

Gephardt, who had said in a speech last week that any politician who "touched" Social Security would be violating "a sacred trust," responded through campaign staff that Simon supported a similar proposal in 1985 to cap Social Security cost-of-living-adjustments (COLAs) as part of broader deficit-reduction package.

One surprise of the 1988 campaign is the way such spats have been insinuated into each party's internal debates at a time when there is no short-term peril to the financial security of the system.

Early in this decade, when the system was in financial trouble, Social Security was often the subject of cross-party sparring, with Democrats accusing Republicans of not wanting to save it.

In 1983, a bipartisan rescue package put the system on solid ground for the near term. But Democratic congressional candidates found in 1984 and 1986 that, by citing an opponent's vote in favor of benefit reduction or COLA cap, they could enhance their standing in polls.

"I don't think there has been another issue that has affected more races in this decade than Social Security," said Lee Atwater, campaign manager for Vice President Bush. "You could make a case it cost the Republicans the Senate in 1986."

One reason that such tactical thrusts are part of presidential campaign politics for the first time is demographic.

Iowa is among the nation's top three states in its percentage of elderly residents. According to estimates from both parties, more than half of the people attending past Iowa caucuses were at least 50 years old.

That percentage could increase this year after an intensive, $700,000 effort by the American Association of Retired Persons (AARP) to get its 300,000 Iowa members to vote in the caucuses Monday night.

The AARP effort includes $400,000 worth of television commercials -- about equal to what the best-funded campaigns here are spending -- 125,000 phone calls and 600,000 letters in the last month. No other issue group is doing as much.

The AARP, however, has chosen to concentrate on long-term health care for the elderly, which has not become what politicians call a "voting issue" largely because all of the candidates agree that more needs to be done.

The Social Security issue appears to be changing votes here and in New Hampshire. Kemp has begun climbing in polls, particularly in New Hampshire, partly because of mailings and ads noting that Senate Republican leader Robert J. Dole (Kan.) led the fight and Bush cast a tie-breaking vote in 1985 to cap Social Security COLA increases. The measure never became law.

Kemp also notes that Dole favors a spending freeze that would, in one of its variations, cap Social Security increases for high-income recipients.

Du Pont is proposing the most fundamental change in Social Security. For two years, he has been advocating that younger workers be permitted to opt of the system partially and fund their retirements through tax-exempt Financial Security Accounts matched dollar-for-dollar with federal contributions.

In this plan, benefits of all current recipients would be protected. But critics charge that breaching the system's universality would end its political support.

Former Arizona governor Bruce Babbitt (D) has called for taxing fully -- instead of by half as is current policy -- Social Security benefits of recipients earning at least $25,000 in family income.

His is part of a broader plan to target all government spending programs to the neediest recipients. Democrats charge that the system would resemble welfare, ultimately eroding middle-class support for it.

Other prominent issues in past presidential contests -- gun control, abortion and arms control -- are almost non-issues this year. On abortion, for example, all Republicans are basically "pro-life," while all Democrats are basically "pro-choice."

Foreign-policy fights that have dominated other GOP nomination fights are nonexistent this year, and economic issues are a chief battleground in both parties.

"The stock-market crash had a major impact on voters' interests," said Jeff Bell, issues director for Kemp. ". . . Since then, people have begun to wonder if the five-year expansion was built on sand."

One impact, Bell noted, was that "advocates of austerity came out of the closet," led by Dole, who has campaigned about the need to take the "bitter medicine" of spending freezes, oil-import fees and, if nothing else works, more taxes, to reduce the deficit.

Among Democrats, Gephardt initially backed away from his tough trade talk after the stock market drop in October. But he resumed it after his poll numbers plummeted.

Trade remains the dominant issue of his campaign. "I've been shocked at how much the trade issue has shaped the dialogue," said William Carrick, Gephardt's campaign manager. "With the exception of Babbitt and taxes, no other candidate is strongly identified with any issue."