TOPEKA, KAN., SEPT. 6 -- President Reagan, heading home to face stiff challenges to his foreign and domestic policies, stopped off here today to wish a happy 100th birthday to Republican Party elder statesman Alfred M. Landon.

" . . . You don't know what a joy it is for a fellow like me to go to a birthday party for someone who in all honesty can call me 'kid,' " the 76-year-old Reagan said in a ceremony on the porch of Landon's three-story white brick mansion.

"In 100 years, Alf Landon has chased many dreams and caught most of them," Reagan said. "Along the way, he's found time to stand for the American values of liberty, democracy and opportunity. And no one is more the living soul of Kansas, which to me means quiet strength and the simple decency of all America."

Then Reagan joined Landon's daughter, Sen. Nancy Landon Kassebaum (R-Kan.), and 150 friends and family members in singing "happy birthday" to Landon, who blew out a single candle and said the president's visit marked "a great day in the life of all of us."

Landon, who will be 100 on Wednesday, attended his first Republican convention in 1912, a year after Reagan was born. He was governor of Kansas for two terms and was the GOP's presidential nominee in 1936, when Reagan was a 25-year-old Iowa sports announcer who cast his vote for President Franklin D. Roosevelt.

In winning the second of his four terms that year, Roosevelt carried every state except Maine and Vermont, an electoral vote landslide unequaled until 1984 when Reagan defeated Democrat Walter F. Mondale and lost only Minnesota and the District of Columbia.

But Reagan's political prospects, which appeared bright after that reelection victory, are highly uncertain in his remaining 16 months in office.

He is under fire from conservatives over a prospective arms control treaty with the Soviet Union and from liberals for his Supreme Court nomination of Judge Robert H. Bork. In Congress he faces a restive Democratic majority anxious to gain control over the nation's economic agenda.

Reagan also bears severe political scars from the Iran-contra affair despite his oft-avowed intention to move on to other issues.

One sign of continuing White House sensitivity to the scandal and the prospect of broad conspiracy indictments against former presidential aides has been Reagan's decision to virtually abandon news conferences. The president last faced the news media in Venice on June 11, and White House officials have advised reporters that another news conference will not be held at least until October.

Instead, Reagan will use the safe forum of a White House speech Tuesday to lay out his remaining agenda.

A senior White House official said last week that a U.S.-Soviet agreement that would scrap medium-range and short-range nuclear missiles in Europe and Asia remains "the highest priority" in foreign policy. Administration officials anticipate that this treaty can be signed at a prospective November summit in Washington between Reagan and Soviet leader Mikhail Gorbachev.

Whether the treaty can be ratified in an election year is another question. White House officials who were once sanguine about ratification prospects now expect protracted opposition in the Senate on verification issues.

Close behind the arms treaty on Reagan's agenda is a still unformed proposal to provide interim aid for the Nicaraguan contras pending a cease-fire and democratic reforms by the leftist Sandinista government of Nicaragua.

"Common sense suggests that the freedom fighters should remain in force as an incentive," White House chief of staff Howard H. Baker Jr. said in an interview last week.

White House officials are working on a proposal to provide the contras with foodstuffs and other supplies after the current appropriation expires Sept. 30. Military aid would be put in a separate account for use only if the Nicaraguan peace plan collapses. Some officials say Congress is likely to insist on a separate subsequent vote on contra military aid.

The president's domestic agenda appears to face even stiffer obstacles. Topping the list of his priorities is the confirmation of Bork, who is being portrayed by numerous liberal and civil rights groups as a conservative activist who would tip the balance of the court.

Baker said last week that he believes Bork can be confirmed but that nearly half the members of the Senate are presently undecided.

While White House officials are relatively optimistic about Bork's chances, they acknowledge privately that there is almost no chance for a budget compromise that would meet deficit-reduction targets. Reagan has made it known, publicly and privately, that he is unwilling to agree to even a small tax increase as part of a compromise package that would also include cuts in defense and domestic spending.

Reagan plans a busy September. He will meet Pope John Paul II in Miami on Thursday, travel to Philadelphia on Sept. 17 for a bicentennial observance and address the U.N. General Assembly on Sept. 21.

"This is a crucial time, and President Reagan will have an active presidency to the end," Baker said.