he US midterm elections have been seen for much of the year as a likely landslide victory for Republicans, with President Joe Biden's approval ratings slumping amid spiraling inflation, record migrant arrivals and rising violent crime.

The Democrats narrowed the polling gap over the summer and were hoping for a much closer contest amid a series of legislative wins and improving gas prices -- but momentum appears to have swung back to the right just before the contests on November 8.

The midterms don't get the attention that presidential elections command, but they are crucial in determining which party has control of Congress -- and the power to advance or frustrate the president's agenda.

All 435 seats in the House of Representatives -- the lower chamber -- are up for grabs, while a third of senators vie for re-election.

The evenly divided 100-member upper chamber -- controlled by Democrats thanks to Vice President Kamala Harris' tie-breaking vote -- is considered the more powerful and prestigious, with its statewide constituencies and six-year terms.

At least eight of the 35 Senate races are considered competitive, but the battle for control of the chamber is likely to come down to five key states.

Pennsylvania

Democrat John Fetterman's commanding lead over Republican celebrity medic Mehmet Oz has all but evaporated, turning the race into a margin-of-error tussle.

Fetterman is recovering from a stroke in May and his shaky performance in the only debate of the campaign has further blunted his momentum.

The pair are duking it out for the seat held by a retiring Republican, in what remains Democrats' top target for flipping a seat.

Democrats have characterized Oz as an opportunistic New Jersey carpet-bagger with tenuous local ties and a penchant for gaffes that demonstrate he is out of touch.

Fetterman, meanwhile, faces scrutiny over his health and has come under fire about his law enforcement record as lieutenant governor, which opponents say was overly lenient.

Nevada

In Nevada, for some time the country's closest race, Republican challenger Adam Laxalt leads Democratic Senator Catherine Cortez Masto by a narrow 0.6 points in the polling average compiled by RealClearPolitics.

Democratic strategists have sounded the alarm over turnout, with many Latinos threatening to sit out the election despite Cortez Masto being the first-ever Latina woman elected to the US Senate.

"It's what's keeping me up at night," Melissa Morales, president of the pro-Cortez Masto Somos PAC, told NBC.

"What I'm looking at is: Do Latinos actually turn out to vote this year? If we see high turnout, we win in Nevada."

The National Republican Senatorial Committee launched Spanish language ad messages in the summer, again focusing on the Democratic candidate's criminal justice record when she was the state's attorney general.

Georgia

In Georgia, Republican challenger Herschel Walker was looking like the Republicans' best bet for a pick-up against incumbent freshman Democrat and pastor Raphael Warnock.

Walker's name recognition as a former football star has kept him in the race despite a series of missteps overshadowing his campaign, and he leads Warnock by 1.4 points in the polling average.

Warnock has focused on cutting prescription drug charges, addressing climate change and helping restore abortion rights.

Walker has focused on the economy and hardline anti-abortion messages.

But his campaign has been beset by allegations of past domestic abuse, an exaggerated resume, fathering children outside of his marriage and paying to have two previous girlfriends' pregnancies terminated.

Ohio

The race to replace retiring Republican Rob Portman was once seen as the Democrats' best opportunity for a pick-up, but Republican J.D. Vance has maintained a consistent if narrow lead since a faltering start to his campaign.

Democrat Tim Ryan -- who has represented the Buckeye State for the last two decades in the US Congress -- has won just two of 15 major polls against Silicon Valley venture capitalist and author, who holds an average two-point lead.

Economic issues top the list of voters' concerns, as they do nationwide, with inflation and the cost of living animating Ohioans the most, followed by abortion.

"It's much better to be a Republican candidate right now than a Democratic one," said Brent Buchanan of polling firm Cygnal.

"Voters are looking at the issue landscape and their candidate options and prioritizing issues that directly affect them, which is driving more undecided voters to the (Republicans)."

Wisconsin

In Wisconsin, Republican Senator Ron Johnson struggled in the summer but pulled ahead of Democratic challenger Mandela Barnes in mid-September and is up by 3.3 points in an average of the last 16 polls.

A recent Fox News poll found that 44 percent of Wisconsin voters think Barnes's political positions are "too extreme" -- against 43 percent for Johnson.

Like Fetterman, Barnes is a lieutenant governor who has been accused of being soft on crime, primarily because of his advocacy for making bail conditions less onerous.

"Under Mandela Barnes, Wisconsin has released 784 violent criminals back into our communities," Johnson tweeted.

"Including 270 murders and attempted murderers. Mandela Barnes' policies make our communities more dangerous. He is too extreme for Wisconsin."

Former president Barack Obama, campaigning for Barnes in the last days of the campaign, blasted Johnson as someone who "understands giving tax breaks for private planes more than he understands making sure that seniors who've worked all their lives are able to retire with dignity and respect."

© Agence France-Presse