The first polling stations opened at 5 a.m. (1000 GMT) in the eastern US state of Vermont for the midterm elections.
Voting is to follow in Connecticut, Indiana, Kentucky, Louisiana, Maine, Missouri, New Jersey, New York and Virginia from 6 a.m. local time.
More than 57 million ballots were cast early in 47 states and Washington DC as of Monday. Many more votes will be cast in person on Tuesday.
Polling stations across the country close between 6 p.m. and 9 p.m., with vote counting beginning immediately afterwards.
Democratic candidates were pushing their message in the key Republican strongholds of Texas, Iowa, Ohio and Kansas as campaigning wrapped upon Monday, seeking to capitalize on voter dissatisfaction with the Iran war and rising prices.
Republicans, meanwhile, cast their opponents as left-wing radicals while touting President Donald Trump's immigration crackdown and tax cuts as policies that will benefit Americans.
In his final public campaign appearance at the National Mall, Trump appeared to lower expectations for his party's performance.
"For some reason, the sitting president doesn't win the midterms, but we gave this a very strong shot," he said, adding that the Democrats had "nominated the most extreme and dangerous slate of candidates in American history."
In rambling remarks, he said little about the national economy, only insisting that it was "great." Instead, he devoted most of his speech to the renovation of Washington's algae-plagued Reflecting Pool.
Vice President JD Vance campaigned in Rome, New York, praising Trump's moves to end taxes on tips and overtime and secure the border while calling Democrats as "lunatics."
In Texas, which hasn't had a Democratic senator since 1988, Democrat James Talarico blamed Trump's "corrupt tariffs" for rising grocery prices and "a new corrupt war in the Middle East" for higher gas and diesel prices.
In the same state, Republican Ken Paxton attacked Talarico in cultural terms as "a vegan who hates meat," and falsely calling him a democratic socialist "just like New York Mayor Mamdani."
Meanwhile, in the key state of Michigan, Democrats were working to maintain a united front after a messy spat that culminated in Democratic Rep. Haley Stevens endorsing Democratic Senate candidate Abdul El-Sayed's Republican opponent, Mike Rogers.
El-Sayed largely ignored the controversy at campaign events on Monday, stressing instead that his campaign was about the concerns of voters.
Stevens did not attend Rogers' rally Monday night, but she got a long ovation from the crowd when Rogers mentioned her endorsement.
The US carried out strikes on Iranian targets in the Strait of Hormuz on the eve of crucialmidterm elections, according to reports.
"Iranians using surface-to-air missiles against US assets" were targeted, a US official was quoted by the Associated Press as saying.
Iranian state television reported that explosions were heard on Qeshm Island, which is located in the Strait of Hormuz.
The US Central Command did not officially confirm the attack.
The strikes came as US President Donald Trump's Republican Party seeks to retain control of Congress in Tuesday's midterm elections. The outcome of the polls will determine how much power Trump has for the remainder of his term.
Trump's popularity has taken a dive, partly in response to the Iran war and its impact on US gas prices. He had previously said the US wouldn't launch new strikes on Iran before the midterms.
The attack came after Tehran said on Monday it had a two-day plan to reopen the Strait of Hormuz and resume nuclear talks with the US if Washington meets its demands, including lifting a blockade of Iranian ports.
Trump wrote on social media on Monday that a deal with Tehran "could happen."
Vice President JD Vance also told reporters that he had spoken to Trump about talks with Iran and said that "the president and I both feel pretty good about the prospects for potentially coming to a negotiated settlement."
