Charles D. (Pug) Ravenel, the 39-year-old Democrat who narrowly missed winning the South Carolina governorship three years ago, announced today that he will run in 1978 for the U.S. Senate seat held by Republican Strom Thurmond.

Ravenel, a former Wall Street investment banker and Harvard Business School graduate, conceded that it will be difficult to beat the 74-year-old Thurmond - a tough campaigner and a political institution since his election to the South Carolina Senate in 1933.

But "like any man or woman who holds office, he can be beaten," Ravenel told reporters at a series of press conferences throughout the state. "If we are right on the issues, we'll win."

Thurmond, who was elected to his fourth full term in 1972 with 63 per cent of the vote, is the only U.S. senator to have been elected as a Democrat, Republican, and write-in candidate. He was elected as an independent candidate on a write-in vote in 1954, and then as a Democrat in 1956. In 1964, Thurmond switched to the Republican Party. He ranks 10th in seniority in the Senate.

Political observers here predict that the Thurmond-Ravenel race will be an exciting contest of classic contrasts: youth vs. age, change vs. status quo. Ravenel is considered to be the underdog.

Ravenel, a native of Charleston, returned to South Carolina in 1970 after a career that included quarterbacking the Harvard football team and working as an investment banker on Wall Street.

Using a sophisticated media campaign, he won the Democratic gubernatorial nomination in 1974 by attacking the state's political establishment. During that campaign, he criticized the state Senate establishment that had blocked legislation on ethics, home rule, judicial reform and government accountability.

But his promising campaign was derailed in September, 1974, when the state Supreme Court, in a suit challenging his eligibility, ruled that Ravenel had not fulfilled the state's five-year residency requirement for gubernatorial candidates.

Then Rep. William Jennings Bryan Dorn was chosen to replace Ravenel as the Democratic gubernatorial nominee. Dorn was defeated by Republican James B. Edwards, who became South Carolina's first GOP governor in a century.

Ravenel's refusal to endorse his replacement - Ravenel said Dorn was the "establishment" candidate and didn't represent what he had been fighting for - left many Democrats bitter and the party split.

Some embittered Democrats may now support Thurmond, it is thought here.

Ravenel made it clear today that he will focus his campaign on Thurmond's record. Thurmond has neither "supported nor sponsored the progressive type of government" that he would like to see, Ravenel said.

President Carter has told Ravenel, who was one of his early supporters that he will campaign for him during the general election. No Democratic primary opposition is expected.

Thurmond already is running hard for re-election. He has raised $124,000 of the $1 million he says his campaign will cost. Last month he moved his wife and four children, aged 18 months, to six years, to columbia for the duration of the campaign.

He is expected to use his young wife and family extensively during the campaign to counteract the age issue.

The presidential standard bearer for the States' Rights Party in 1948. Thurmond also has been moving to moderate his racial image in a state where the electorate is now 26 per cent black. He has become well known in the black community for procuring grants and loans to black colleges and in 1976. Thurmond sponsored a black, Matthew Perry, for a judgeship on the U.S. Military Court of Appeals.