Michael Dukakis could win the presidency by honing his appeal to the sizable number of undecided voters, but the stability of his opponent's lead makes the task a formidable one, pollsters say.

Two polls released Monday found Republican George Bush maintaining a healthy lead among likely voters - 11 points in one survey done last week, seven points in a Harris poll conducted during the weekend.The Harris poll found Dukakis gaining a bit by countering Bush's attacks of his record on crime. Fifty-three percent rated Dukakis as soft on crime, still high but down from 63 percent in an earlier poll by Harris.

Dukakis also appeared to score slightly by portraying Bush's campaign advertisements as unfair: While 68 percent called Dukakis' own advertising too negative, a higher 75 percent said that of Bush's ads.

Overall, Harris put the race at 52-45 percent, a very slight narrowing from 53-44 in its Oct. 14-17 poll. The contest was 52-41 in the other new poll, done by Gallup for the Times Mirror Corp. the first half of last week.

The Times Mirror poll found support for Dukakis uneven within his own party, while Bush has solid backing from Republicans and siphoned away some Democrats. Gallup, in its analysis of the results, noted that Bush has led the race at least since September.

Though the contest remained largely stable, the Times Mirror poll found that Bush had increasingly loyal support in key groups. Dukakis' popularity eroded, meanwhile, and he lost support in some Democratic groups even while gaining it in others.

The problem in analyzing the race was Bush's stable lead on one hand versus the weakness of voter preferences on the other. Twenty-seven percent of likely voters were undecided or said they might change their minds, and only about half of each candidates' supporters backed them strongly.

"If Dukakis and the Dukakis campaign can get their act together and speak to their constituencies or potential constituencies, they can win it on the basis of the numbers," said Andrew Kohut, the president of Gallup. "But that's a big if, given the course of the campaign so far."

Bush's lead was based upon overwhelming support from Republicans, solid backing from Republican-leaning independents and "substantial defections" from some Democratic groups, according to Kohut's analysis.

"For Dukakis, on the other hand, his support is not as strong among his own partisans; and he is receiving virtually no defections from members of core Republican groups," the analysis said.

The Times Mirror poll divides the electorate into 10 evenly sized groups whose members share general values. Its "partisan poor," for example, is a Democratic group; its "enterprisers" and "moralists" are Republican.

Bush attracted 21 percent of the "partisan poor," a good showing in a Democratic stronghold. Dukakis, by contrast, drew only 2 percent of the "enterprisers."

"Disaffected" voters, a potential swing group whose members are set apart by their suspicion of government, continued to back Bush by more than 2-1.

"In contrast, on the Democratic side, defections to George Bush have doubled among `seculars' and significantly increased among the `partisan poor,"' Gallup's analysis said.

While Dukakis offset the losses by gaining with other Democrats, the movement was "indicative of the problems that the Dukakis campaign has had in achieving enthusiastic support" from groups in his own party, it said.