The Conservatives responded that they felt their use of the footage in the advertisements fell within fair dealing doctrine in Canada, pointing out that when they ran an advertisement in 2009 using footage from C-SPAN, the director of C-SPAN responded to the Liberal Party's notification by saying that the Conservative use of their footage fell within fair use guidelines.
The problem with the Conservatives' response is that in many ways it's not really relevant to the legal issue at hand. Ignoring for the moment that C-SPAN and the CBC both have the right to determine what their own response should be when their footage is used without permission (regardless of whether that response is legally correct), it is worth remembering that Canada and the United States operate under differing copyright regimes.
There is no explicit exemption for political speech in either Title 17 of the United States Code (the statutory source of copyright in the United States) or in the Copyright Act in Canada. From a statutory standpoint, Canada only allows fair dealing exemptions for use of copyrighted material when the intended use is for research, public study, criticism/review, or news reporting.
Unlike the United States, which explicitly outlines other factors by which a given use of copyright material could be considered a fair use, in Canada most people accused of copyright infringement and pleading a fair dealing defense must rely on CCH v. Law Society of Upper Canada, which outlines a set of similar factors, including among others the purpose of the dealing, the character of the dealing, the nature of the original work and the effect of the dealing on the original work. Although this common-law precedent has been used in Canada for much the same purpose as the statutory fair use provisions in place in the United States, a common-law precedent is by definition more transient than a statute.
Which then leads one to the other problem with the Conservative Party's response to the CBC, which is not simply that they are claiming a fair dealing right that does not exist under statute, but that they're claiming a fair dealing right which the Conservatives specifically did not include in Bill C-32, the bill updating outdated provisions copyright law - which they wrote.