Presidential contender Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE immediately conceded the New Hampshire primary after media organizations quickly called the race for Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) on Tuesday.

The Clinton campaign conceded in a memo and said it was setting its sights on upcoming races in South Carolina and Nevada. Both are states where she holds a substantial lead.

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"After splitting the first two contests, an outcome we've long anticipated, attention will inevitably focus on the next two of the 'early four states," Clinton campaign manager Robby Mook wrote in the memo.

"The nomination will very likely be won in March, not February, and we believe that Hillary Clinton is well positioned to build a strong – potentially insurmountable – delegate lead next month."

The Associated Press, MSNBC and CNN all called the race for Sanders as the final polls closed at 8 p.m. ET. The AP had only reported about 10 percent of the vote, with Sanders leading by a margin of 56 percent to 42 percent.

Sanders's win in New Hampshire had been expected — he came into Tuesday's primary with a substantial lead in the polls.

The bigger question for Democrats centers on his margin of victory.

The Sanders camp hopes that a definitive win can catapult him and narrow the gaps in both South Carolina and Nevada, where he trails the former first lady with minority voters. The Clinton team hopes to blunt his momentum by keeping the margin close.

The Clinton memo points at her "high levels of support in the African American and Hispanic communities" as proof that she has the edge over Sanders in a protracted Democratic primary fight.

It also argues that while Clinton has been in the public spotlight for years, media scrutiny of Sanders will only increase as the contest moves forward.

—This story was updated at 8:37 p.m.