Political campaign spending by special-interest groups shot to an all-time high of $349.6 million in the last election, the Federal Election Commission said Saturday.

Outlays by 4,828 political action committees, many of them formed by special-interest groups that lobby Capitol Hill, rose 5 percent in the last two years over its mark in the comparable 1985-86 period, the commission said.That did represent a slowing of growth in campaign spending by special interest groups, which previously had been charted at 22 percent, the FEC said.

Democrats, who control both houses of Congress, benefit most from the generosity of the special interests, the commission said. The groups tend to favor incumbents and incumbents tend to win re-election.

Political action committees, known as PACs, are formed by corporations, labor unions and other organizations to raise money for political contributions. Under federal law, a PAC may give each candidate a maximum of $5,000 for each primary, runoff and general election.

Individual contributions are limited to $1,000 each for primaries, runoffs and general elections.

In 1987-88, $1.2 billion overall was spent on campaigns for Congress and president, including money from the federal government, individuals, political parties and PACs. Of that total, the PACs accounted for about 28 percent, according to commission figures.

During 1987-88, the PACs put 74 percent of their money on incumbents, while in the previous two-year period 68 percent went to incumbents, the commission said. Challengers reaped 12 percent of the PAC money in the last election compared to 14 percent in 1986.