Who is right? Answer: Neither. Global output will rise under non-selective protectionism (or has an expansionary bias, to be more precise). Protectionism reduces the propensity to import. That doesn’t mean imports will fall. Total imports of an individual nation will rise because of higher income. World trade will rise because of higher world income.
In other words, non-selective protectionism acts by reducing the propensity to import by price elasticity effects but raises volume of imports via income elasticity effects.
The world as a whole is balance-of-payments constrained, not just individual nations. Raising tariffs on imports incentivises producers to produce more as they will face less competition from abroad. Consumers will shift to domestically produced goods because of price elasticity effects.
Moreover, since governments of most nations won’t have a balance-of-payments constraint if there are large tariffs, they will be free to boost domestic demand by fiscal policy, limited only by the economy’s capacity to produce. If it is done, it will be a conscious behaviour by the government.
There is of course another way fiscal policy gets relaxed because of balance of payments. Reduction of current account deficits, relative to gdp, reduces the budget deficit, relative to gdp (as can be shown by a behavioural model and this shouldn’t be surprising as the two are related by an accounting identity). Typically governments follow some rules even if they aren’t explicitly required and their expenditure is endogenous to the government budget deficit: they tighten fiscal policy when the budget deficit goes out of a limit and relax fiscal policy when the budget deficit is within the limit. So improvement in a country’s balance of payments position would lead to a relaxation of fiscal policy, automatically.
To summarize, protectionism if done the right way can raise world trade because of rise in world income. There is no economic case against protectionism. There is opposition because few corporations want to increase their share in world markets. Protectionism reduces share of these mega corporations instead of reducing world trade. So “free trade” (which is managed trade for a few) only benefits a few and imposes a huge cost on the world economy.
All that is for the current world economic outlook. Typically in deep recessions governments take protectionist measures. In such scenarios, since output is falling, there is a tendency to confuse this with causation. It is more accurate to say that protectionism prevented a deeper implosion in such cases.