President Reagan, in a campaign-ending media blitz aimed at making best use of his broadcast communication skills, is to urge Americans in national and regional television advertisements next weekend to "stick to the task before us" by electing Republicans on Nov. 2.

"They don't call him the Great Communicator for nothing," one administration official said in explaining why Reagan will tape two five-minute spots for the $500,000 TV blitz beginning Saturday in 60 regional markets. White House spokesman Larry Speakes said the spots will be used on network television at 10:55 p.m. Sunday and Monday.

"Stick to the task before us, let's get the job done," Reagan is to say in one of the spots, which are variations on his basic campaign theme of "stay the course."

The White House also announced yesterday that Reagan will make his first trip to South America, beginning Nov. 30, the day after Congress is scheduled to return for a lame-duck session. The president is to visit Brazil, Colombia and Costa Rica on a five-day trip.

Reagan, who will be campaigning in North Carolina today and in four Rocky Mountain states Thursday and Friday, will not make any more appearances in the final three days before next Tuesday's election, in which Republican candidates appear increasingly vulnerable. Instead, he will turn to pretaped television, which reaches far more people with less wear and tear and political risk for the president.

The television spots are not intended to break new ground. Instead, they repeat his 1980 campaign message of "saving the American dream" and "making America well again" and repeat the president's frequent contention that he cannot "undo the damage" of the last 20 years in 21 months.

Reagan's national television appeal emphasizes the importance he places on the midterm elections, in which the working coalition of Republicans and conservative Democrats who have supported his economic programs in the House may be at stake.

Former president Gerald R. Ford breakfasted with Reagan at the White House yesterday and afterward took a bullish view of both the economy and the elections, predicting that Republicans would lose "give or take 15" seats in the House. Senior White House officials say they believe that the president could preserve a working majority with a 15-seat loss.

Ford, who has been campaigning for GOP candidates, also said that Reagan's "basic popularity is very solid" and that his economic program is beginning to work.

"When you have a cold and the sniffles, you can take aspirin and you get cured," Ford said.

"But when you have pneumonia, you have to have strong medicine, and it takes more time to get results."

Despite Ford's optimism, the administration is clearly worried about the elections. Reagan's advisers say they believe it is worth spending some of the president's political capital in an across-the-board appeal for Republicans on television in the hope of averting a landslide loss that could imperil his ability to govern effectively.

Today, Reagan is to visit North Carolina, one of the few states where Republican challengers are considered to have a reasonable chance of defeating Democratic incumbents or winning open seats. The president will speak at a rally in Raleigh for four House nominees and at a separate event to award singer Kate Smith the Medal of Freedom.

On Thursday he is to travel to the friendly confines of Wyoming, Montana and Nevada, and finish his presidential travel with a visit to New Mexico on Friday.

Reagan's trip to Brazil, the sixth the president has made out of the country, is in response to a state visit May 12 by Brazilian President Joao Figueiredo.

A senior administration official stressed the need for inter-hemispheric security in the wake of the Falklands war. He said the United States and Brazil share "free-enterprise, anti-communist" values and "fundamental mutual economic interests."

The official said that relations had improved during the last two years after concern about human-rights violations and a dispute over nuclear safeguards during the Carter administration.

In another action yesterday, the president signed legislation introduced by Rep. Michael D. Barnes (D-Md.) that will provide incentive grants to states to adopt stringent measures to deter drunk driving.