Democrats are holding onto the advantage in voters' loyalty they regained two years ago. The two political parties had moved close to even in strength during President Reagan's first term. In a Gallup poll released today, 42 percent of voters describe themselves as Democrats, 29 percent as Republicans and 29 percent as independents.

In early 1985, after Reagan's huge reelection victory, the Republicans held a significant advantage among whites, 18- to 29-year-olds, college-educated and affluent groups and midwesterners. The GOP was at a standoff among male and female voters, high school graduates, skilled blue-collar workers, in nonunion households and in the West. The surge brought the GOP closest to parity with the Democrats for the first time since 1946.

Today, Republicans hold no significant advantage in any major population group and are at parity only among the college-educated, business and professional people and those whose family income is $40,000 or more.

Gallup said that as Reagan's influence has dwindled, many of the Democrats he attracted in 1984 have returned to the party fold. By the summer of 1985, the Democrats had recaptured a 7 percentage-point advantage, and the gap has slowly widened.