In recent days, we have seen evidence of the willingness by Donald Trump Jr. and others in the Trump campaign to obtain information damaging to Hillary Clinton from the Russian government — information that is as probative to Mr. Mueller's investigation as it is threatening to the president. Congress must ensure that Mr. Mueller can continue his investigation independently, effectively, and steadfastly. Should Mr. Mueller be fired, the current majority must understand they have a duty to enforce our system of checks and balances — as we did in 1973. If Republicans cannot appreciate that obligation, then the American people will be forced to decide whether they really want a Congress that continues to ignore its responsibility.