Taking an early lead in the Gallup Poll in a test election against Vice President George Bush, Massachusetts Gov. Michael Dukakis--the probable Democratic presidential nominee--has two important advantages over his party's 1984 standard-bearer, Walter Mondale.

First, Dukakis faces an opponent who lacks the broad personal popularity enjoyed by Ronald Reagan four years ago. Voter opinions of George Bush are mixed, with only around half expressing favorable views. Second, Dukakis has credibility among the voters on economic issues and taxes that Mondale, who promised a tax increase, lacked throughout most of the 1984 race.Voters prefer Dukakis over Bush on the issue of the federal deficit, Dukakis at least equals Bush's rating on tax policy. Gallup's analysis shows that Bush's negatives and Dukakis' ability to hold his own on the economy, an area of Democratic weakness in recent election, have played a key role in turning the "swing vote" Dukakis' way.

With two-thirds (66 percent) of the electorate in the "swing vote" category--only moderately supporting one of the candidates or still undecided--the potential for shifts in voter sentiment between now and November remains high.

Swing voters represent a group more likely to place themselves squarely in the middle of the ideological spectrum and to call themselves political independents.

At this point, swing voters are less likely to make distinctions between Dukakis and Bush on the issues. Therefore, general image of the two candidates takes on greater importance in explaining candidate preferences of this group. Right now, Dukakis has a big advantage over Bush in overall favorability among swing voters.

Opinion of Dukakis is overwhelmingly positive (69 percent favorable vs. 16 percent unfavorable), while opinion of Bush is mixed (49 percent favorable vs. 42 percent unfavorable).

The significance of Bush's high negatives among swing voters is seen in the following statistic candidate preferences among swing voters with an unfavorable view of Bush break down 87 percent Dukakis to 6 percent Bush. Moreover, 30 percent of swing voters are now expressing anti-Bush sentiments in their choice of candidates, a greater proportion than either the pro-Dukakis (19 percent) or pro-Bush vote (22 percent). Dukakis, who now draws few negatives, receives an anti-vote of only 9 percent among swing voters.

As analysis of how the two candidates stack up on the issues among swing voters suggests that Dukakis has been able to win over many voters on an area that has been one of the Republicans' greatest strengths--the economy. The same analysis suggests, however, that since the issues likely to matter most among swing voters have been used to GOP advantage in previous elections, they could yet break Bush's way as the campaign progresses.

To put the importance of issues to swing voters in perspective, Gallup ranked 11 issues on the basis of how large a differential exists between the proportion of respondents who feel closer to their preferred candidate on the issue and the proportion who feel closer to their preferred candidate's opponent. At the top of the list of issues important to swing voters is the federal deficit (40 percentage points) national defense (39 percentage points) and tax policy (39 percentage points). Just below the top of the list is fighting drugs (36 percentage points).

Of the top three issues to swing voters, Bush holds a clear advantage on one--national defense. By a 40 percent to 27 percent margin, swing voters prefer Bush over Dukakis on defense. On a second issue, tax policy, the two candidates are close--38 percent feel closer to Dukadis while 27 percent feel closer to Bush. Of the three issues, Dukadis's only clear advantage is on the federal deficit (36 percent vs. 25 percent).

Dukadis's advantage in the economic area among swing voters is critical because the issue areas where he has the largest advantage over Bush--the homeless, day-care, the environment, health care, etc.--rate further down the list in their importance to swing voters.

In effect, the issues that are most important to swing voters bear a closer resemblance to the issues of greatest interest to Republican loyalists than those of greatest interest to Democratic faithfuls.