Literally speaking the Conservative’s slogan doesn’t assert anything; it’s a rhetorical question which doesn’t even seek an answer. But it is a very suggestive question: “are you thinking what I’m thinking?” is the kind of thing you might say to someone before jumping into bed with them. In itself, it doesn’t put forward a proposition, but it does indicate the possibility of a symmetrical reflection of views.
All it needs is some contextual clue as to what the topic of agreement might be. So for one of their campaign posters, the Tory’s main text was: “It’s not racist to put limits on immigration,” written in a hand-writing font, and underneath in a smaller typed font they reproduced their slogan. The choice of font is meant to give the impression that this is the view of an “ordinary” person, a non-politician; the poster becomes a metaphorical looking-glass which reflects public opinion. And the party slogan is merely an invitation for you to agree with what other ordinary people think. It’s a very sophisticated rhetorical technique which is deserving of further analysis. And of course it doesn’t have to involve drawing attention to itself with a rhetorical question; if anything that probably weakens the whole effect. But its most interesting aspect can be seen when it is deployed as part of a strategy to influence public debate to your party’s political advantage.
The ploy comes down to this: make a statement that in itself is almost trivially true, but which in the broader context of political discourse could be associated with an extreme position on a particular issue (say, anti-immigration). The opposing side of this issue is then faced with an unpalatable choice: it either responds by denouncing the statement, which leaves them vulnerable to the charge of contradicting common sense and being outside the mainstream; or it either doesn’t respond or agrees with the literal content of the statement, either of which can be interpreted politically as giving tacit support to the other side. The opposition therefore finds itself in a double bind. (But also note that this strategy is less likely to be effective when attempted by the political opposition against a government, as e.g. was the case in Britain, because the government has a third option: neutralising an issue by making changes to public policy.)

