WASHINGTON — After waging an 18-month assault on the Republican establishment, President-elect Donald J. Trump changed course on Tuesday and enlisted the party’s high priests of foreign policy to help him win the confirmation of Rex W. Tillerson as secretary of state.

Several former Republican secretaries of defense and state sought to dismiss bipartisan concerns about Mr. Tillerson, the Exxon Mobil chief executive, over his two-decade relationship with President Vladimir V. Putin of Russia. At the center of the debate are questions about Mr. Tillerson’s vocal opposition to American sanctions imposed on Russia as he pursued oil and gas deals in that country.

Their mobilization showed how much Mr. Tillerson’s nomination is already a Congressional proxy fight over Mr. Trump’s embrace of Russia and Mr. Putin. Democrats issued blunt denunciations of the idea that a globe-trotting energy executive could adequately represent the national interests of the United States. So did several leading Republicans, whose party orthodoxy has been anti-Kremlin for decades.

“I have serious concerns about his nomination,” said Senator Marco Rubio, Republican of Florida. Senator Ben Cardin, Democrat of Maryland, declared himself “deeply troubled” by Mr. Tillerson’s opposition to sanctions imposed by the United States on Russia after its intervention in Ukraine in 2014.