laboring oar

Noun

 * 1) An oar that is worked with great effort.
 * 2)  The part of a task that requires the greatest effort to achieve.
 * 3) * 2019, Kisor v. Wilkie (U.S. Supreme Court No. 18-15), Justice Kagan:
 * And so too, when new issues demanding new policy calls come up within that scheme, Congress presumably wants the same agency, rather than any court, to take the laboring oar.