Rubio: We're Headed Toward A Series Of 'Grave' Defense Policy Errors

"Just read Gen. Mattis resignation letter. It makes it abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation,damage our alliances & empower our adversaries," Rubio tweeted shortly following Trump's announcement that Mattis would be stepping down in February.

"I hope we who have supported this administrations initiatives over the last two years can persuade the President to choose a different direction. But we must also fulfill our constitutional duty to conduct oversight over the policies of the executive branch," the Florida Republican wrote. 

Rubio, a 2016 GOP presidential candidate, had earlier praised Mattis, a former four-star Marine general, for serving the country "with great distinction."