DES MOINES, NOV. 15 -- Sen. Paul Simon (Ill.) has moved to the top of the Democratic presidential field in Iowa, while Senate Minority Leader Robert J. Dole (Kan.) holds a narrow lead over Vice President Bush in the GOP race, the Des Moines Register poll indicated today.

The poll said Dole was favored by 36 percent of those likely to attend the Feb. 8 Republican precinct caucuses, compared with 30 percent for Bush. The rest of the GOP field trailed far behind.

Among Democrats, Simon received the backing of 24 percent of those who said they will attend Democratic caucuses, with Massachusetts Gov. Michael S. Dukakis coming in second at 18 percent. Rep. Richard A. Gephardt (Mo.), who led in the last poll, published in August, was third with 14 percent.

"A lot of little things are breaking in the right direction," said Simon during a campaign stop in Minnesota.

Gephardt played down the poll's results, saying, "I don't want to peak in November, I want to peak on Feb. 8 in Iowa."

Dukakis said he was generally satisfied with a second-place finish, but said the poll "doesn't mean much."

Questioned during an Iowa campaign stop, Dole said the poll confirmed his feeling that the GOP battle is a two-person race.

Simon held third place in the last poll the newspaper published, but since then has scored some major organizational gains. When Sen. Joseph R. Biden (Del.) dropped out of the race, Simon picked up the bulk of his large, experienced staff, and has campaigned heavily in the state.

Jesse L. Jackson was fourth among Democrats with 11 percent; former Arizona governor Bruce Babbitt was fifth with 8 percent, and Sen. Albert Gore Jr. (Tenn.) was last with 3 percent, the poll indicated. Gore announced on Friday he was pulling most of his staff out of Iowa to concentrate on other tests in New Hampshire and the South.

Rep. Jack Kemp (N.Y.) was in third place among Republicans at 9 percent, followed by former television evangelist Marion G. (Pat) Robertson at 8 percent, former Delaware governor Pierre S. (Pete) du Pont IV at 5 percent and former secretary of state Alexander M. Haig Jr. at 2 percent.

The newspaper said it conducted a telephone survey Oct. 29-Nov. 12 among 312 Republicans and 330 Democrats who said they "definitely or probably" would attend the Iowa caucuses.