Jason Reed / Reuters

Sen. John Kerry waves at the end of his speech during the final session of the Democratic National Convention in Charlotte, N.C., on September 6.

Updated at 7:01 a.m. ET: When he ran for president, many in the GOP slammed Sen. John Kerry, D-Mass., pilloried for his late opposition to the Vietnam War and his famed flip on the conflict in Iraq. But, as criticism mounted against U.N. Ambassador Susan Rice as the apparent frontrunner to become the next secretary of State, Kerry was publicly embraced by Republican colleagues in the Senate as a comparatively slam-dunk candidate to replace Hillary Clinton.

Now that Rice has withdrawn her nomination to the post, as NBC News reported exclusively on Thursday, all eyes turn to the onetime Democratic nominee. An official close to the process told NBC's Andrea Mitchell late Thursday that Kerry is now almost certain to get the job. "There were two people on the list," the person said. "Two minus one is one." 

In her withdrawal letter to the president, Rice said she was convinced her nomination would prove "lengthy, disruptive and costly" as Republicans have raised questions about her role in the public response to the 9/11 attack on the U.S. consulate in Benghazi, Libya.  The exclusive NBC News interview with Rice aired Thursday on Rock Center with Brian Williams.

In a statement, Kerry praised Rice as an "extraordinarily capable and dedicated public servant" and alluded to his own past political battles.

"As someone who has weathered my share of political attacks and understands on a personal level just how difficult politics can be, I've felt for her throughout these last difficult weeks, but I also know that she will continue to serve with great passion and distinction," he said. 

EXCLUSIVE: Rice drops out of running for secretary of state

Elected to the Senate in 1984, Kerry rose to national prominence as a foreign policy expert when he returned to the Senate after his failed 2004 presidential bid. The chairman of the Foreign Relations Committee since 2009, he has made high-profile visits to Afghanistan and Pakistan and helped negotiate the new arms treaty with Russia that was signed in 2010.

Respected in the upper chamber and nationally as a shaper of the nation's foreign policy, Republicans have indicated that Kerry would face little opposition to be confirmed to the secretary of State post. "I think John Kerry would be an excellent appointment and would be easily confirmed by his colleagues," said Republican Susan Collins, R-Maine, late last month. Sen. Rob Portman of Ohio, a close ally of former Republican presidential nominee Mitt Romney, declared that Kerry would have "an easy time" being confirmed in the Senate. 

Kerry's confirmation would likely not come without some minor re-litigation of past controversies.  One of Congress's richest members, he was painted as an out-of-touch patrician by his presidential foes. The onetime Navy lieutenant was criticized by opponents during his campaign for his high-profile protests of the Vietnam War, including his nationally-covered challenge to a congressional panel in 1971 to defend the deaths of men "for a mistake."

Kerry worked closely with the president in the just-finished election, playing Romney in debate preparations and had been seen as a potential choice to head either the State Department or the Department of Defense. Earlier today a top Pentagon official told NBC News that former Republican Senator Chuck Hagel appeared to be the likely choice for secretary of Defense.

But the main headache for Democrats if Kerry is appointed will be the triggering of a special election in Massachusetts next year to replace him. Democrats recently celebrated the ousting of Republican Sen. Scott Brown, who won a January 2010 special election to replace the late Sen. Ted Kennedy. Elizabeth Warren bested Brown in the heavily blue state by a margin of 54 percent to Brown's 46 percent.

If Kerry is picked, Brown will be viewed as a formidable Republican candidate to replace him. A wider bench of Democrats, including former Senate candidate Martha Coakley, may vie for the nomination.

But whoever wins the potential replacement race would have a grueling path, as would voters weary of statewide contests. Another special election would be the state's second in three years, and Kerry's successor would be up for re-election again in 2014.