Meanwhile, some Democrat supporters admitted they were haunted by the spectre of 2016 when Mr Trump's triumph confounded the pundits and opinion polls which had predicted a Hillary Clinton victory.
In Georgia, Stacey Abrams, 44, the former minority leader of the state House of Representatives, is running for governor against Republican Brian Kemp, the 55-year-old Georgia secretary of state.
In the United States, during what's been one of the most remarkable political seasons in the country's modern history, some experts are wondering if the country is in the midst of a historic partisan realignment, one that could have lasting repercussions on the traditional red-blue model. All of his big achievements - like tax reform and the appointment of two US Supreme Court judges - have been extremely hard fought.
The races for governor in Florida and Georgia are also too close to call.
Two races which are of particular interest are Georgia, where Democrat Stacey Abrams is seeking to become the first black woman to become governor of a U.S. state, and Florida, where Democrat Andrew Gillum is locked in a bitter battle with Republican and Trump acolyte Ron DeSantis.
In Michigan, Gretchen Whitmer, a prosecutor and former state senator, won her race, flipping that state.
Obama has taken on a more public role this fall after refraining from offering a full-blown counterpoint to Trump's policies, which have sought to dismantle Obama's legacy.
USA stocks ticked higher in thin trading on Tuesday, as investors awaited the election results. This time, Sen. Dianne Feinstein has coasted against Democratic state Sen. But if Republicans do better than expected, Trump will be able to claim vindication for his campaign strategy and his unconventional approach to the presidency.
Four of those seven California battlegrounds are in Orange County, once considered Republican heartland but where much has changed due to demographic shifts over the past two decades.
In what could be a warning for Republicans, the NBC poll reported college-educated white women - the so-called suburban moms - favour Democrats by a substantial 61% to 33%. More women than ever were running, along with military veterans and minorities, many of them motivated by Trump's rise. "We are being dwarfed by the Democrats". That has been the loud and clear message of many Democratic candidates.
In Pittsburgh, where residents just finished burying those gunned down at the Tree of Life synagogue, some voters saw their Election Day decisions as a way to send a message that the country is headed down a dark and risky path.
That state's Democratic senator, Bill Nelson, also appeared to be looking at a narrow defeat at the hands of Republican opponent Rick Scott, with all signs pointing towards the GOP actually widening its Senate majority: Democratic Sen.
In Florida, Democrat Debbie Mucarsel-Powell, an immigrant from Ecuador and educator, focused her campaign largely on health care and toppled Rep. Carlos Curbelo, the Republican incumbent. "When people can just lie with abandon, democracy can't work", he told a cheering crowd as he campaigned for Senator Joe Donnelly.
"There's got to be consequences when people don't tell the truth, when words stop meaning anything". "But I can tell you I would not be calling Homeland Security, the Federal Bureau of Investigation and the GBI unless we had information that we needed them to look at".
It was the first national election in the United States since the 2016 race in which Russian Federation interfered, according to US intelligence agencies, with a campaign of hacking and propaganda to sow discord, harm the Democratic presidential candidate and boost Trump's chances.
While men with military backgrounds have always been recruited to run for office, this year's candidates include several female veterans. Groups for or against the amendments raised well over $20 million, according to campaign finance reports.
New Jersey's Senate race was not expected to be close, but it may be. Trump had won that district handily.
The Associated Press called the very first US House race at 6:32pm, when Republican Hal Rogers safely won re-election in Kentucky's fifth congressional district.
"It's a year that could be good for Dems", said Jon Thompson, a spokesman for the Republican Governors Association.
Sensing that his Republican base favors the hardline stand, Trump has sent soldiers to the Mexican border, threatened to have illegal immigrants shot if they throw stones at the border, and vowed to restrict citizenship rights.
A record 33 of the Tuesday's matchups for Congress were women vs. women.
In Virginia, political newcomer Jennifer Wexton defeated two-term GOP Rep. Barbara Comstock.
"Are women fired up?"
Tuesday's elections also tested the strength of a Trump-era political realignment defined by evolving divisions among voters by race, gender, and especially education.