The need for contrast allows some usually non-accented elements of the verbal phrase to be accented. This is the case with the finite operator do, for instance, as we can see in the following example.
The last IP is in narrow focus. The idea or having to leave has already been stated, so the speaker accents didn’t, which otherwise would be unaccented, in order to express his opposition to it.
The last three examples refer to very well known structures in English where the operator do and the copula be are accented more for emphasis than contrast.
(53) That is alarming (David Hyde Pierce; New York, US).
(54) This is a surprise (Groucho Marx; New York, US).