The Republican Party lost a 1984 Senate candidate but gained a likely 1988 presidential contender yesterday in Delaware Gov. Pierre S. du Pont IV.

Du Pont, 48, a three-term House member before being elected to the first of two terms as governor in 1976, announced that he would not challenge Sen. Joseph R. Biden Jr. (D-Del.) next year. His statement said he wanted to focus his efforts on what he called "the post-Reagan" agenda for government, and friends said he has his eye on the 1988 presidential race.

Du Pont's decision not to tackle Biden--the best Democratic vote-getter in recent Delaware history--was a blow to the GOP, which is defending a shaky Senate majority in 1984.

It also was part of a pattern that has seen governors of both parties reject opportunities to run for the Senate, which in the past has been an alluring goal for ambitious state politicians.

Tennessee Gov. Lamar Alexander (R) has repeatedly turned down pleas from his party's Washington leadership that he seek the seat of retiring Senate Majority Leader Howard H. Baker Jr. (R-Tenn.). With Alexander out, Rep. Albert Gore Jr. (D-Tenn.) is the early favorite to pick up that seat.

Republicans had hoped du Pont might give them an offsetting gain by challenging Biden, a two-term veteran. Published polls showed the two men evenly matched.

Biden issued a statement saying du Pont's decision was "welcome," because "obviously he would have been a very formidable opponent. I like him, and it would have been a tossup race." Though Biden said he still planned to run "a serious race," Republicans acknowledged that they have no strong challenger in view.

Earlier this year, Democrats had struck out on attempts to enlist two of their popular governors as 1984 Senate candidates. South Carolina Gov. Richard W. Riley (D) said he would not challenge Sen. Strom Thurmond (R-S.C.), and Colorado Gov. Richard D. Lamm (D) rejected strong pressure to oppose Sen. William L. Armstrong (R-Colo.).

Two other Democratic governors reportedly have not yet turned down pleas that they get into the Senate races in their states. Maine Gov. Joseph E. Brennan (D) is being sought as an opponent for Sen. William S. Cohen (R-Maine), and Mississippi Gov. William F. Winter (D), whose term ends this year, as a challenger to Sen. Thad Cochran (R-Miss.).

In West Virginia, where veteran Sen. Jennings Randolph (D) is stepping down, retiring Gov. John D. Rockefeller IV is expected to be the Democratic candidate. Republicans hope to enlist former governor Arch A. Moore Jr. as their nominee. Moore has a 1-1 record against Rockefeller in gubernatorial races.

The reasons other governors turn down Senate races vary from case to case, but apparently the Senate has lost some allure for upwardly mobile politicians, particularly those with their eye on the presidency.

"Being a senator is a high honor and an important responsibility," du Pont said in a formal statement. "But it is a full-time job by itself. Its very nature forces attention to the issues of the day rather than the agenda of tomorrow."

Du Pont also said he had enjoyed being an executive far more than a legislator. "Legislators weigh needs, resolve conflicts, enact compromises," he said. "A legislator reacts to the agenda. The executive branch manages, organizes and makes things happen. An executive sets the agenda."

Hinting strongly at his future plans, du Pont said that, "beginning in 1985, I will have the opportunity, the interest and the time to consider and prepare for America's agenda for 1988 and beyond."