Sen. Richard Schweiker (R-Pa.) said yesterday he will not seek a third Senate term in 1980 and will work actively in support of the presidential campaign of former California governor Ronald Reagan.

Schweiker was Reagan's choice for the Republican vice presidential nomination in 1976.

Asked if he would again run with Reagan if asked in 1980, Schweiker said, "It is presumptuous to run for vice president."

Schweiker was also asked if he might seek the 1980 GOP presidential nomination if Reagan dropped out.

"I will cross that bridge when I come to it," Schweiker said.

There has been some speculation that Reagan may not run for reasons of health or age.

Schweiker said, "I am personally convinced he is going to run for office."

Schweiker, 52, was first elected to the House in 1960, after defeating a Republican incumbent in a primary election.

He won election to the Senate in 1968, defeating incumbent Democrat Joseph Clark.

He was perceived as a Republican liberal in the Senate until 1976, when Reagan unexpectedly tapped him as his running mate before the Republican presidential nomination had been decided.

Since 1976, Schweiker has increasingly voted along conservative lines. His approval rating by the liberal Americans for Democratic Action plummeted from 80 to 20 percent.

Schweiker said he has made no plans for his future beyond the end of his present six-year Senate term. projection of 1975. This means geologists are convinced that the least amount of undiscovered oil in the