In the aftermath of the Democratic National Convention, the party's nominee, Michael S. Dukakis, has expanded his lead among registered voters over Vice President Bush, the probable Republican nominee, according to a Gallup Poll.

This was among the findings of a national public opinion poll of 948 registered voters conducted late last week for Newsweek magazine by the Gallup Organization. The telephone interviews took place on July 21, which was the last night of the convention, and on the night after that.

Fifty-five percent of the 948 registered voters interviewed in the poll said they preferred to see Mr. Dukakis win the 1988 Presidential election, while 38 percent said they preferred to see Mr. Bush win. The poll had a margin of sampling error of plus or minus four percentage points.

This represented a shift in Mr. Dukakis's lead from the 47 percent to 41 percent advantage he held in the last pre-convention Gallup Poll, taken by telephone July 8-10. In that poll, 1,001 registered voters were interviewed.