Los Angeles — * John Glenn, and not Walter Mondale, would give Ronald Reagan the toughest contest in next fall's presidential election.
* But Mr. Mondale will be the Democratic candidate.
* Foreign affairs will be the key to the election.
* And without a nuclear arms accord with the Soviets, President Reagan will face a tight battle.
These are the perceptions Reagan campaign strategist Stuart K. Spencer offers on the presidential race.
Mr. Mondale will beat Senator Glenn for the Democratic nomination, in Mr. Spencer's view. But Mr. Glenn, as Democratic nominee, would put Ohio and some of the Southern states out of the President's reach, while they are ''winnable'' for the President against Mondale.
Spencer, a southern Californian and longtime political adviser to the President, was - as Reagan campaigners phrase it - ''de facto political director'' of Reagan's 1980 campaign and is ''one of the most prestigious advisers and strategists'' for the 1984 campaign.
He foresees a very tight presidential race next fall. His comments were made before the Democratic candidate debates in New Hampshire.
There are two Spencer scenarios. One is that the economy improves or simply holds its ground and Mr. Reagan achieves an arms agreement with the Soviets. In this case, Reagan wins handily. The other is that the economy simply holds its ground and there is no arms agreement. In this case, the race is close.
The latter scenario is the most likely, he says. ''I don't think the Russians are going to do Ronald Reagan any favors'' - such as grant an arms agreement that would brighten his reelection prospects.
Foreign affairs will be the key to the election, according to Spencer. Nuclear arms control will be the overriding issue, but events in the Mideast and Central America could also flare up. ''They're all explosive. They all could make or break the President.''
As the presidential campaign heats up, the President himself will be out campaigning about three days a week, Spencer says. While security will be tighter than it was in 1980, with some ''dampening effect'' on spontaneity, it is still politically important for the President to get out of Washington and onto the campaign trail.
Spencer also implied that Reagan would not try to duck a debate with the Democratic nominee. ''Debates have become institutional. I don't think you can avoid them.''
Texas is the most crucial swing state for Reagan next fall, Spencer says. Of 8 key states - California, Texas, Illinois, Ohio, Michigan, Pennsylvania, New York, and New Jersey - the President will need 3, preferably 4, to win.
If Reagan is reelected, Spencer notes, both Chief of Staff James A. Baker III and Michael K. Deaver have indicated they want out of their present jobs. Mr. Baker is seeking a Cabinet post, such as attorney general or secretary of state, and, according to Spencer, is likely to get one.
Will controversial former Interior Secretary James G. Watt be an active fund-raiser for Republicans? Spencer is asked.
''Jim Who?'' he replies.