Coulter's comment is the latest in the Republican's conservative movement's refusal to support McCain, whose campaign is gaining strength heading into next week's Super Tuesday.
"If you are looking at substance rather than if there is an 'R' or a 'D' after his name, manifestly, if he's our candidate, then Hillary is going to be our girl, because she's more conservative than he is," Coulter said Thursday on Fox's "Hannity and Colmes." "I think she would be stronger on the war on terrorism."
Coulter took McCain to task for several positions, such as his anti-torture stance, saying he and Clinton weren't that far apart.
The commentator also said she's prepared to campaign on Clinton's behalf should McCain become the Republican presidential nominee.
"John McCain is not only bad for Republicanism, which he definitely is, he is bad for the country," she said.
Coulter is the latest conservative commentator to speak against McCain. Talk-radio host Rush Limbaugh said McCain's popularity came from a "fractured" conservative base and an "uninspiring" GOP presidential field.