Senator Bernie Sanders, the independent self-declared socialist from Vermont who challenged Hillary Clinton in the Democratic presidential primaries, made his debut in party leadership, as chair of outreach, a sign that the bruised Democrats have learned from Clinton's defeat in last week's presidential election.
In all, the 48 Democrats elected a team of 10 senators tasked with defining party strategy until the mid-term elections of November 2018.
Liberal firebrand Senator Elizabeth Warren is also part of leadership, as vice chair of the Democratic conference.
"We are ready to stand shoulder to shoulder with Republicans -- working with soon-to-be President Trump on issues where we agree," Schumer, who replaces retiring Senate minority leader Harry Reid, said in a statement after the internal elections.
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"But we will go toe to toe against the president-elect whenever our values or the progress we've made is under assault."
Despite Republicans controlling both chambers of Congress, Trump will need to deal with the Democrats in the Senate, where the chamber's rules grant strong powers of obstruction to the minority party.
A seat for a senator from Louisiana will be decided in a runoff election December 10.
Schumer, a 65-year-old New Yorker known for his abrasive personality, is a shrewd political strategist capable of forging compromises with opponents but also able to launch harsh attacks to achieve his ends.
Republican Senator Lindsey Graham said he expected "the good, the bad and the ugly" out of Schumer.
"The good: he's a dealmaker. The bad: he can throw a punch. When it comes to politics, he knows how to play it, he's from New York, he can get kind of ugly."
Republican House Speaker Paul Ryan congratulated McConnell, but also Schumer, saying: "We look forward to working with him constructively for the good of the country.