Cats are obligate carnivores, without meat they will die, dogs are not obligate carnivores, but they do best on a carnivorous diet.
They are able to digest some amount of starch, but due to the absense of salivary amylase all this digestion is done by pancreatic amylase and excess starch can overburden the pancreas leading to pancreatitis, insulin resistance and eventually diabetes.
Their shorter GI tract (though longer than a cat) is best able to digest meat and fat.
they are certainly able to survive on a diet with less meat and more plant material, but they do not thrive on it and since their bodies produce their own carbs by converting protein and fat, they don't need any in the diet at all.
A typical wolf diet would be about 14% carbs (from fruit and veg rather than grain) and with the genetic changes in dogs they can cope with maybe 20-25% carbs in the diet reasonably well, but not the 50% found in many dog foods.
I see evidence of this freqently with dogs switching to a high meat content food and owners are amazed at the improvement in their condition, even though they already believed they were in a good condition. Provide the right building blocks and the body will generally take care of itself.
the nutrients in foods are generally used for core processes and muscles first and once these have been satisfied any remaining nutrients can be used by the extremities, ie skin, hair and nails, and that is why the improvements are generally so noticeable. Many toxins are often also disposed of through hair shedding so some poorer foods also create additional moulting.
Also interesting is that the main change made for a specific puppy or senior food is to increase the protein content as the adult foods often don't have enough protein for those life stages, where with a high meat content, high protein food the nutrient levels are already sufficiently high and one food can be suitable from weaning through adult to old age, surely suggesting the more carnivorous diet is better.
So my opinion is that they are Carnivores, but with a limited omnivorous capacity.
Also interesting is that the main change made for a specific puppy or senior food is to increase the protein content as the adult foods often don't have enough protein for those life stages