Andrew Harnik, Associated Press
Republican Presidential candidate and Senate Foreign Relations Committee member, Sen. Marco Rubio, R-Fla., departs a briefing on Iran nuclear negotiations, Tuesday, April 14, 2015, on Capitol Hill in Washington, Tuesday, April 14, 2015.

WASHINGTON — Sen. Marco Rubio's fiery criticism of an emerging deal over Iran's nuclear ambitions is fizzling in the cool pragmatism of the Senate.

Rubio joined colleagues on Tuesday in backing a compromise measure during his first full day as a presidential candidate. The Florida Republican is running head-first into the awkward conflict between campaigning for the White House in absolutes and serving on an influential panel that often conducts its business in compromise.

Rubio returned to Washington to vote for legislation that that would give Congress the power to approve or reject any agreement with Iran over its nuclear programs.

Rubio unsuccessfully sought a long-shot demand that Iran back Israel's right to exist in exchange for eased economic sanctions. Yet as senators considered the issue, Rubio backed off.