Although the presidential race is neck-and-neck between President Carter and Ronald Reagan, Carter is clearly the favorite among the fifth of the electorate who can be described as evangelicals, a group whose vote could be decisive in a close election.

In a trial heat among the three leading presidential candidates, Carter receives the support of 52 percent of evangelicals who are registered to vote to 31 percent for Reagan and 6 percent for John B. Anderson, the independent.

In contrast, among the nonevangelical segment of the population, Carter and Reagan are in close contention. The GOP challenger receives 39 percent of the vote to 36 percent for Carter and 15 percent for Anderson.

Further evidence of the pro-Carter leanings of evangelicals is seen in their rating of the president's performance in office. Among evangelicals 46 percent approve while 29 percent of nonevangelicals do so.

One of the unique aspects of the 1980 presidential race is the fact that all three of the leading contenders -- Carter, Reagan and Anderson -- consider themselves "born-again" evangelical Christians.

On balance, being identified as an evangelical is more a political asset than a liability. A large majority of nonevalengicals (78 percent) indicate that their preference is not influenced by whether a presidential candidate is an evangelical or have no opinion on the issue.

In contrast, the views of the nation's evangelicals are decidedly proevangelical, with 55 percent saying they would be more likely to vote for such a candidate, only 2 percent against. A total of 43 percent say it would make no difference or have no opinion.

Nationally, roughly twice as many (19 percent) say they would be more likely to vote for an evangelical presidential nominee as they say they would be less likely to vote for him (9 percent).

A profile of evangelicals shows that certain groups are overrepresented in their ranks: women, nonwhites, persons with less than a college education, southerners, older people, protestants, rural residents and the less well-to-do.

The percentages of evangelicals currently registered to vote, and who indicate a likelihood of voting, closely parallel the peracentages for nonevangelicals.

Evangelicals tend to be politically more Democratic than nonevangelicals, and at the same time more consaervative ideologically. Nearly six in 10 (57 percent of evangelicals describe themselves as Democrats compared to 47 percent for nonevangelicals. And in response to the question of which party they would like to see win in their congressional district, 63 percent of evangelicals say the Democrats as compared to 56 percent of nonevangelicals.

In addition to the Democratic leanings of evangelicals, also helping to boost Carter's standing is the fact that many more voters (51 percent) are aware that the president considers himself a born-again Christian than know that Reagan and Anderson also place themselves in the evangelical wing of Christianity. Only 14 percent and 11 percent, respectively, know that Reagan and Anderson consider themselves evangelical Christians.