Elections were held in the United States on November 6, 2018. These midterm elections occurred during incumbent Republican president Donald Trump's first term. Although the Republican Party increased its majority in the Senate, Democratic incumbents and challengers vastly outperformed Trump's margin in Republican-leaning states, and unified Republican control of Congress and the White House was brought to an end when the Democratic Party won control of the House of Representatives. In what was widely characterized as a "blue wave" election, Democrats also gained governorships, other statewide offices, and state legislative chambers.

Democrats made a net gain of 40 seats in the United States House of Representatives, gaining a majority in the chamber and thereby ending the federal trifecta that the Republican Party had established in the 2016 elections. The Republican Party retained control of the United States Senate, making a net gain of two seats and defeating four Democratic incumbents in states that had voted for Trump in 2016. As a result of the 2018 elections, the 116th United States Congress became the first Congress since the 99th United States Congress (elected in 1984) in which the Democrats controlled the U.S. House of Representatives and the Republicans controlled the U.S. Senate. In state-level elections, Democrats picked up a net of seven governorships and several state legislative seats.

This was the first time since 1970 that one party gained Senate seats while losing House seats, which had also occurred in 1914 and 1962, and would go on to also happen in 2020, 2022, and 2024 as well. In the state elections, Democrats gained seven state governorships, control of approximately 350 state legislative seats, and control of six state legislative chambers.

The elections marked the highest voter turnout seen in midterm elections since 1914, at 49.4%. The elections saw several electoral firsts for women, racial minorities, and LGBT candidates, including the election of the first openly gay governor and the first openly bisexual U.S. senator. In various referendums, numerous states voted to expand Medicaid coverage, require voter identification, establish independent redistricting commissions, legalize marijuana, repeal felony disenfranchisement laws and enact other proposals. During the campaign, Democrats focused on health care, frequently attacking Republicans for supporting repeal of provisions of the Affordable Care Act (also known as Obamacare), including protections for individuals with preexisting conditions. They also focused on tying many Republican incumbents and candidates to President Trump. Republican messaging focused on immigration and the Tax Cuts and Jobs Act of 2017. There were allegations of attempted Russian interference in these elections as well as controversies regarding potential voter suppression.

Research has linked Republican losses in the elections to the party's unsuccessful and unpopular efforts to repeal the Affordable Care Act, as well as the China–United States trade war.

In May 2018, President Trump began to emphasize his effort to overcome the traditional strength of the non-presidential party in midterm elections, with the "top priority for the White House [being to hold] the Republican majority in the Senate". He was already well into his own 2020 reelection campaign, having launched it on his inauguration day in January 2017. By early August, the president's midterm efforts had included rallies in Ohio, Pennsylvania, Florida, Montana and elsewhere "reprising the style and rhetoric of his 2016 campaign". He focused his message on the economy, his proposed border wall, the "trade war" with China, criticism of the media, and his proposal to create the space force, a new branch of the military devoted to operations in space. In late August 2018, the Huffington Post reported that Trump and his administration had been engaging in campaign activity on taxpayer-funded trips. According to the report, a top White House staffer identified 35 events by Cabinet and senior staff members "with or affecting House districts in August already". White House Deputy Press Secretary Lindsay Walters called the report "misleading".

The 2018 elections featured a wider range and larger number of campaign advertisements than past midterm elections. Almost a third of Republican ads focused on taxes, especially on the recently enacted Tax Cuts and Jobs Act of 2017. By mid-October 2018, at a cost of some $124 million, more than 280,000 television advertisements related to immigration had been aired in House, Senate and gubernatorial races, representing a five-fold increase compared to the 2014 cycle. In October 2018, The New York Times and The Washington Post characterized Republicans' 2018 campaign messaging as being chiefly focused on fear-mongering about immigration and race. According to The Washington Post, President Trump "settled on a strategy of fear—laced with falsehoods and racially tinged rhetoric—to help lift his party to victory in the coming midterms, part of a broader effort to energize Republican voters". In November 2018, Facebook, NBC, and Fox News withdrew a controversial pro-Trump advertisement that focused on a migrant caravan; Facebook noted that the ad violated Facebook's rules concerning "sensational content".

Nearly half of all advertisements by Democrats focused on health care, in particular on defending the Patient Protection and Affordable Care Act (also known as Obamacare or the Affordable Care Act) and keeping in place protections for individuals with preexisting conditions. A number of Republican candidates claimed to support provisions of the Affordable Care Act, such as protections for preexisting conditions, even though they supported efforts that either weakened or eliminated those provisions. In the final weeks of the campaign, Democrats indicated their desire to keep the focus of the campaign on Republican efforts to repeal provisions of Obamacare through the proposed American Health Care Act of 2017. A Gallup poll conducted days before the election found that voters considered healthcare and the economy to be the top issues among registered voters, though many voters also considered immigration to be a top priority.