It's hard to get a handle on whether Trump's tariffs are going to be a long-term feature of his economic policy or whether they are primarily a negotiating ploy that will be dropped in return for reciprocal concessions from foreign countries. I tend to take Trump at his word when he says he's for free trade but he wants reciprocity, but it's not clear to me that everyone in his administration has the same view of it. An explanation from Trump — I don't really want these tariffs in place for a long time, they are just a negotiating ploy — would undermine their value as a negotiating ploy, which may be why he hasn't provided such an explanation.
As for the senators and congressmen denouncing the actions — people such as Paul Ryan and Pat Toomey and Ben Sasse and Mitch McConnell — it's entirely within their Article I power under the Constitution to zero out these tariffs or eliminate them or repeal the law that delegates to the president the power to impose them. Unless and until they do that, their criticism of the president on the issue just looks like empty posturing. Senator Toomey says he'll cosponsor Senator Mike Lee's bill to take back some of Congress's tariff authority. That bill — Lee's "Global Trade Accountability Act of 2017" — is currently backed by Senators Flake, Gardner, and Sasse, so there is a long way to go to get a Senate majority behind it. A related House measure, H.R. 5281, the "Global Trade Accountability Act of 2018," introduced by Rep. Warren Davidson of Ohio, has backing from about a dozen Republican Congressmen.