The Democrats were slightly favored to wrest control of the U.S. Senate from the Republicans in Tuesday's elections, according to some analysts, with the outcome helping to determine how difficult it will be for the next president to pass legislation.

Democrats' hopes for big gains in the Senate and the House of Representatives, both now controlled by Republicans, were tempered in the closing days of the 2016 campaign, even if Democratic nominee Hillary Clinton wins the White House.

Two weeks ago, Democrats hoped to sharply reduce the Republicans' 246-seat House majority and grab control of the Senate. But the FBI may have dashed those ambitions by reigniting a controversy about Clinton's emails while she was secretary of state, congressional aides and analysts said.