There has been a surge of early voting for the US midterm elections, with millions more Americans going to the polls compared with the last such election amid a series of tight contests that will determine control of Congress.

More than 50.5m early votes have been cast across the US as of Saturday morning, according to an analysis by the University of Florida. This is already nearly 5m more votes than the total number of early votes made in the last midterm elections, in 2022.

Voter enthusiasm for Tuesday’s election appears to be strongest among Democrats, with 10.7 million registered Democrats having voted so far, according to the states that collate such information, compared with 7.8 million registered Republicans.

These party affiliations are not indicative of actual votes cast but polling has shown widespread unhappiness with the presidency of Donald Trump and unusually close races in states usually won comfortably by Republicans.

Republicans are defending a 53-47 Senate majority in a midterm election in which the battleground map has expanded significantly, with Democrats aiming to pull off victories in traditionally conservative-leaning states such as Texas, Alaska and even Kansas that could allow them to take control of the Senate.

In Texas, a state central to Democratic ambitions, more than 7 million people have already voted early, University of Florida data shows. This is already 1.5 million more people than the number who voted early in the state during the 2022 midterms.

In Harris county, which includes Houston, 862,700 people had voted as of Wednesday, an Associated Press analysis found, compared with 564,500 at the same point in 2022.

The surge comes in a county that has reliably voted Democratic in recent elections, though by varying margins. In 2024, Kamala Harris defeated Trump there by nearly 6% in the presidential contest, while Democratic Senate candidate Colin Allred carried the county by roughly 11% over the Republican incumbent senator Ted Cruz.

This year, the state’s Republican attorney general, Ken Paxton, faces Democrat James Talarico in a closely watched Senate race, while the Republican governor, Greg Abbott, is seeking re-election against Democrat Gina Hinojosa.

Hundreds of millions of dollars have poured into the Senate race as Republicans fight to defend a state where Democrats have not won a statewide election in more than three decades. Trump visited Texas earlier this week, his third trip there in a month.

In other close-fought Senate races, early voting has also increased in these midterms, according to the AP. In South Carolina, which Trump won by nearly 18% in the 2024 president election, early voting is up 83% compared with the 2022 midterms.

In Kansas, where another unusually tight Senate battle is under way in a normally reliable Republican state, early voting is up 34%, while in Maine early voting is up 44%. In Michigan, the site of another key Senate race, early voting is up 32%.

The midterms will decide the composition of the US House of Representatives and Senate for the final two years of Trump’s second term, with the rising costs of food, gasoline and other essentials, as well as the Iran war and concern over government corruption, forming the backdrop to the voting.

Trump, weighed down by deep unpopularity among voters, has sought to boost struggling Republican candidates with rallies held across various previously safe states. This effort has included the US president commenting repeatedly on the physical appearance of candidates and dismissing alarm over rising gasoline costs as secondary to the profits of fossil fuel companies.

Trump hinted at the toil of this effort at a rally in Florida on Friday night. “I thought this was it,” he said. “I thought I could relax. Twenty-eight days of this crap in a row is a lot. It’s a lot.”