The Federal Election Commission has officially approved the use of credit cards to make contributions to political candidates.

In an advisory opinion handed down Thursday, the FEC told the campaign committee of Illinois Democratic Senate candidate Alex Seith that its decision "assumes that the credit-card issuers . . . will follow their normal collection procedures with respect to obtaining payment from persons who used their credit cards to make political contributions."

For political fund-raisers, the credit card could prove more popular than [WORD ILLEGIBLE] cash because the donor wouldn't have to pay the bill for about a month. And unlike with other types of pledges, the money could be collected by the candidate from the bank before the credit-card company even bills the donor.

The Seith campaign had cooked up an idea to make fund-raising a little easier. Seith radio and television commercial would end with a toll-free telephone number and donors would be invited to call and charge their contribution on their Visa or Master Charge cards.

Although the Democratic Party telethons in 1974 and 1976 allowed donors to charge contributions, Seith campaign officials decided they would not go ahead without FEC approval.

The FEC said that the costs of processing the phone contributions and charges from the card issuers for collecting the money would be considered expenditures and have to be reported.

A Seith aide said yesterday that, even with the FEC [WORD ILLEGIBLE] , he was not certain the credit-card fund-raising method would be used in the campaign.