One Republican political strategist surveying his party's prospects in the November elections is reminded of the old pickup chugging and wheezing up a steep mountain road with a bumper sticker that reads: "I may be slow, but I'm ahead of you."

Despite the economic problems that increasing numbers of voters are blaming on President Reagan and the Republican Party, he and many of his colleagues think about the number of House and Senate seats they expect the GOP to lose in November with remarkable equanimity.

He estimates the Republican loss of House seats well on the sunny side of 20. He won't be surprised if the GOP gains a Senate seat or two, and in his heart he wouldn't be surprised if it turns out even better in both houses.

At that, he's a doom sayer compared with Nancy Sinnott, executive director of the National Republican Congressional Committee, who won't necessarily concede any loss: "I don't think we'll fall much below the 192 seats we have now," she says.

The reasons why they think their truck is ahead of the Democrats:

* Their enormous advantage in money and direct mail, which they contend will enable them to dominate the political dialogue in September and October. They plan a continuous $6 million to $7 million television ad campaign this fall, which they hope, as in 1980, will influence late deciders, who in recent elections have favored GOP candidates.

* Strong Republican candidates, partly because of the party's assurances of plenty of financial and technical aid. This includes the freshman incumbents, who, like the Democrats' class of 1974 "Watergate babies," understand the advantages of incumbency and the new political techniques.

* The fact that the voters perceive that the Democrats offer no alternatives except to return to their traditional programs of the past 50 years. The Republicans' campaign theme will play on their success in convincing the voters in 1980 that it was "Time for a Change" after years of alleged Democratic mismanagement of the economy, and hopefully convince the voters that the Republicans have the country started on the right, albeit difficult, economic track.

* Reagan's ability, through the powers of the presidency and his strong powers of persuasion, to frame and define the issues in the context of his agenda of basic changes he's trying to effect. A recent poll by Richard Wirthlin, the White House pollster, show that 48 percent of the respondents blame Jimmy Carter and the Democrats for the recession, while only 19 percent blame Reagan and the Republicans, and that a strong majority believe that Congress has passed less than half of Reagan's program.

Other polls indicate that while people think they're worse off now than they were a year ago they still expect things to be better a year from now.

* Predictions of a low turnout -- as low as one-third of the eligible voters according to a recent census survey -- which generally helps Republicans, who vote in greater proportions.

Despite the uproar over their TV ad showing the folksy mailman delivering Social Security checks with the 7 percent cost-of-living increase, the Republican National Committee is showing it in 19 major markets again starting this week.

"Regardless of the news and free media, we can totally dominate the political dialogue with paid TV," says one GOP strategist. "The Democrats screamed about that ad, but the Teeter and Harris polls showed there was a big turnover in people's attitudes, about 30 percent in Reagan's favor, toward the two parties on Social Security."

What the polls revealed was that because of the ad campaign, there was an increase of 20 percent of those who thought Reagan had raised Social Security benefits -- the increase was legislated before he took office -- and a drop of 8 percent of those who thought he had cut benefits.

The Democrats have an analysis of the GOP's national campaign on behalf of their candidates in 1980, which ruefully concedes the Republicans' "sheer technical expertise" as well as the technical and financial resources of their national committees and individual candidates.

One of these resources is direct mail. The Democrats' study quotes a RNC survey that concludes that "the most powerful communication tool in 1980 was direct mail . . . . When the voter had no exposure to television, but was exposed only to Republican direct mail, the Republican candidate enjoys a 15 percent variance to the expected vote."

The survey shows that about one-third of all voters in 1980 made up their minds in the last few days of the campaign.

They are the least interested and least likely to vote and the Republicans' money for television and direct mail heavily influenced them.

As they did in 1978, the very late deciders "inevitably help Republican candidates," the survey concludes.

The optimism of Sinnott and other strategists is based partly on their confidence in the reelectability of their 52 House freshmen, 37 of whom won Democratic seats and who traditionally are the most vulnerable incumbents.

"Our freshman class is like their Watergate class of 1974," Sinnott says. "They understand the media, the techniques, and how to use their incumbency."

A Democratic strategist who predicts a 15- to 20-seat Democratic gain, agrees.

"The Republicans have good candidates and the Democrats don't have enough strong challengers," he says. "You need twice as many strong challengers as the number of seats you hope to gain and we just don't have that many."

The Democrats argue that as the effects of unemployment, recession and Reagan budget cuts weigh in, their prospects get better and better and are limited only by their lack of money.

"We have enough good issues that we don't have a totally bad climate," responds a Republican strategist. "We're going against 40 years of mismanagement but we've got the economy headed the right direction. Inflation, interest rates, taxes, government spending and regulations are going down.

"The ads will make the point that this election is of historical importance to maintain the change of direction people voted for in 1980."