President Donald Trump's slumping opinion poll numbers over his response to the coronavirus pandemic are weighing on Republicans chances of holding on to their 53-47 majority in the US Senate.
If Democrat Joe Biden wins the White House, Democrats would need to win at least three Republican-held seats for a majority, giving his running mate Kamala Harris a tie-breaking senate vote. If Mr Biden loses, Democrats will need to turn four Republican seats for a senate majority.
One of the key Senate races is Arizona where Republican senator Martha McSally trails Democratic challenger Mark Kelly by just over 11 percentage points in opinion polls.
Ms McSally, a former US representative and air force combat pilot, was appointed to the seat once held by the late Republican Senator John McCain after losing her 2018 Senate bid to Democrat Kyrsten Sinema. Mr Kelly, a former astronaut and navy combat pilot, has been leading Ms McSally in opinion polls for more than a year.
In Colorado, Republican senator Cory Gardner, a former US representative who entered the Senate in 2015, is now among the most vulnerable senate Republicans, partly because of his allegiance to Mr Trump, in a state that has gone Democratic in the last three presidential elections.
Democrat John Hickenlooper, a former two-term governor and 2020 presidential hopeful, is leading Mr Gardner in opinion polls. But the Democrat's polling lead has slipped from double digits in July to just five percentage points, raising the chances that Mr Gardner could pull through.
In Georgia, first-term Republican senator David Perdue, a wealthy businessman who promotes himself as a Trump ally, leads Democratic challenger Jon Ossoff by an average of four percentage points in opinion polls. He also holds a fundraising advantage.
Mr Ossoff, an investigative journalist and media executive, ran a powerful campaign for a US House of Representatives special election in 2017, but ultimately lost. Democrats believe he could oust Mr Perdue, partly as a result of public disaffection for Mr Trump's response to the Covid-19 pandemic that has hit Georgia hard.
In Iowa Democrat Theresa Greenfield, an urban planner and real estate developer, is running neck-and-neck against Republican senator Joni Ernst at a time when Iowa is experiencing a spike in coronavirus infections. Ms Greenfield has accused Ms Ernst of being a rubber stamp for Mr Trump and not taking the Covid-19 pandemic seriously enough. Ms Ernst, a first-term US senator with a cash advantage over her opponent, labels Ms Greenfield as hostile to police and says her campaign is bankrolled by out-of-state liberals.
In Maine, four-term Republican senator Susan Collins, a New England moderate long known for her independence, has seen her popularity flag among voters amid criticism that she has failed to be a moderating force in the Senate during Mr Trump's presidency.
Her Democratic challenger, Maine House of Representatives Speaker Sara Gideon, has led Ms Collins by several points for months in opinion polls and has out-fundraised the Republican. Polls suggests the race could be tightening, with Ms Gideon ahead by only a single percentage point.