PREVIEW-South Africa's ANC majority under threat in poll

* ANC's 70 percent majority may be reduced

* Zuma to become president

* Future economic policy key for investors

By Marius Bosch

JOHANNESBURG, April 20 (Reuters) - South Africans vote on Wednesday in the closest election since the end of apartheid with the ruling African National Congress' huge majority likely to be cut by a revitalised opposition.

ANC leader Jacob Zuma will become South Africa's fourth democratic president but will have his work cut out to assure investors his government will stick to policies which created the country's longest period of economic growth.

More than 23 million voters are registered to cast their ballots on Wednesday in national and provincial elections in Africa's biggest economy.

Analysts say the ANC's 70 percent majority may be reduced to between 60 and 65 percent -- removing the ability to change the constitution at will with a two-thirds parliamentary majority.