There are two competing factors at play. The first is that Doug Ford is not his brother. Rob Ford has managed to maintain a degree of sympathy with a segment of the population despite all of the issues of the last year. Doug Ford, on the other hand, has no such well of sympathy.
There have not been many polls conducted related to the possibility of a Doug Ford candidacy. But a few polls have asked about him. The most recent survey, a poll conducted by Forum Research in May, put Doug Ford at only 20% if he replaced his brother. This was at a time when Rob Ford was polling at 24%.
Rob Ford's approval rating at the time was 32%, one of the lowest levels he has ever recorded. Doug Ford's approval rating in the same poll was 30%.
In a poll conducted by Forum in November 2013, Doug Ford was similarly polling below his brother.
And in an Ipsos Reid poll from the same month, 34% of respondents said they trusted Rob Ford whereas only 30% said the same about Doug Ford. To be fair, however, Doug Ford's approval rating in that poll was two points higher than Rob's (42% to 40%).
So this suggests that we might expect, all things being equal, that Doug Ford would poll below his brother's level of support, which is currently averaging 29%.
The second factor at play, however, is that Doug Ford will not be replacing his brother in a vacuum. Though Rob Ford has withdrawn from the mayoral race, he has put his name on the ballot to be a councilor. Doug's candidacy, then, is almost as a proxy for Rob. This is the Ford family running for the job, rather than Doug alone. Some voters who liked Rob but not his brother may still vote for Doug Ford because of this. Add the extra sympathy that Rob Ford's health problems might give him, and it has the potential to boost the kind of numbers Doug Ford would have had on his own.
So perhaps the next set of polls will show little difference, with Doug merely being slotted in for Rob. But there are seven weeks left to go in the campaign, and Doug is not the campaigner his brother is and has had testy exchanges with the media in the past. He will have debates to attend as well. If he manages to maintain Rob Ford's support out of sympathy at first, he may have difficulty holding on to it through to October 27.