After a divisive campaign marked by fierce clashes over race, immigration and other cultural issues, Americans are casting votes to determine the balance of power in the US Congress and shape the future of Donald Trump's presidency.
The first national elections since Mr Trump captured the White House in a stunning 2016 upset are a referendum on the polarising president and a test of whether Democrats can turn the energy of the liberal anti-Trump resistance into victories at the ballot box.
The Democrats have a good chance of winning at least the US House of Representatives, but slimmer hopes of gaining control of the Senate, opinion polls show.
If they do take the House, Democrats could launch congressional investigations into aspects of Mr Trump's administration, from his tax returns to possible conflicts of interest, challenge his overtures to Saudi Arabia, Russia and North Korea, and oppose him on immigration, tax cuts and trade.
If Republicans hold both chambers of Congress, Mr Trump would likely claim vindication for his polarising style, a month after he secured a conservative majority on the Supreme Court when the Senate approved nominee Brett Kavanaugh after a fight over sexual misconduct accusations that split the nation.
Striking a dark tone at a rally in Indiana yesterday evening, Mr Trump accused Democrats without offering any evidence of "openly encouraging millions of illegal aliens to break our laws, violate our borders and overrun our country."
All 435 seats in the House, 35 Senate seats and 36 governorships are up for grabs in elections focused on dozens of competitive races that opinion polls show could go either way.
Democrats must pick up a minimum of 23 House seats they need for a majority.
US stocks ticked higher in thin trading on Tuesday, as investors awaited the election results.
Problems with voting machines were preventing some Americans from casting ballots in a dozen states, US rights advocates said.
That follows complaints about registration problems, faulty equipment and intimidation they have received throughout early balloting.
A Department of Homeland Security official said the reports of voting technology failures appeared so far to have had no significant impact in preventing people from voting.
Voter turnout in national elections, normally lower when the White House is not at stake, could be the highest for a mid-term election in 50 years, experts predicted.
About 40 million early votes were likely cast, said Michael McDonald, a professor at the University of Florida who tracks the figures.
In the last such congressional elections in 2014, there were 27.5 million early votes.
At least 64 House races remain competitive, according to a Reuters analysis of the three top non-partisan forecasters, and Senate control was expected to come down to a half dozen close contests in Arizona, Nevada, Missouri, North Dakota, Indiana and Florida.
In his time in office, Mr Trump has pushed tax cuts through Congress and overseen a period of economic and jobs growth but has failed so far to deliver on presidential campaign promises to replace the Obamacare healthcare law and build a wall on the Mexican border against illegal immigration.
A Democratic victory in the House would further hinder the border wall plan and complicate congressional approval of a deal to update the North American Free Trade Agreement.
Mr Trump also could face more pushback from Democrats on trade tariffs he has introduced, particularly in farm states hard hit by retaliatory measures from China or manufacturing states hit by higher steel and aluminum prices.
Wrapping up the campaign in recent days, Mr Trump repeatedly raised fears about immigrants, issuing harsh warnings about a caravan of Central American migrants that is moving slowly through Mexico toward the US border.