Arkansas Gov. Bill Clinton declared yesterday that he will not run for the 1988 Democratic presidential nomination, upstaging what had been a heavily promoted news conference today at which he was widely expected to declare his intention to run.

The decison came as a surprise to both prospective allies and adversaries, who noted that Clinton had begun the process of hiring staff experienced in national campaigns, contacting major fund-raisers and traveling to such key states as Iowa and New Hampshire.

"My head has told me to enter this race, but my heart says no," Clinton, who has repeatedly signaled his interest in national politics, said in a three-page statement. In making the announcement, Clinton joined fellow Arkansan Sen. Dale Bumpers (D-Ark.) in rejecting a 1988 bid for the presidency, along with Sen. Bill Bradley (D-N.J.) and New York Gov. Mario M. Cuomo.

Clinton, a former Rhodes scholar, cited the fact that over the past 13 years he has been a candidate in 15 campaigns. "After the 1986 election, Hillary {his wife} and I looked forward to a more orderly life" with the opportunity to devote more time to his daughter, Chelsea, 7, he said.

In addition, Clinton noted that he had been elected to the first four-year gubernatorial term allowed in Arkansas in over a century and contended that he "could best serve the people of Arkansas by continuing to do the governor's job with all the creativity and energy I possess."

Clinton's decision not to run leaves only one southerner, Sen. Albert Gore Jr. (D-Tenn.), in the fight for the Democratic presidential nomination. Sen. Sam Nunn (D-Ga.) has said he does not intend to run but will make a decision at the end of summer. The schedule of primaries has been changed for 1988 to place much more importance on the South. Fourteen states in the region and surrounding states will hold Democratic primaries next March 8, shortly after the New Hampshire primary and Iowa caucuses.

Few campaign professionals ascribed any large change in politics of the Democratic field because of Clinton's absence.

Paul Tully, a principal organizer for the campaign of Massachusetts Gov. Michael S. Dukakis, said Clinton faced major hurdles getting into a presidential race at this late date. "No matter how good you are, there is a long time frame to build the money" and to line up the kind of support needed to win delegates, Tully said.

Rep. Beryl Anthony Jr. (D-Ark.), chairman of the Democratic Congressional Campaign Committee, said Clinton's decision frees him to support the presidential bid of Rep. Richard A. Gephardt (D-Mo.).