The states that could swing control of the Senate from the Republicans to Democrats

There are 34 states up for Senate elections on Tuesday. The Democrats need to win 14 of them.

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As well as making their choice between Hillary Clinton and Donald Trump, voters in 34 states will also elect their representatives in the United States Senate next Tuesday. At the moment, the Republicans have control of the upper chamber, with 54 of the 100 Senators.

But Republicans are on the defensive this year, and not just because their presidential nominee is Donald Trump. The Senate is elected in thirds every two years, meaning that the 34 seats up this time were last contested in 2010. Those were midterm elections – where lower turnout tends to benefit Republicans – at a time when President Obama was less popular than he is now. Holding on to a number of the seats they won then will be harder at these elections.

Of the 66 seats not up for election this time, Democrats hold 36 (including the two independent Senators who caucus with them: Bernie Sanders and Angus King). That means they need to win 14 of this year’s 34 races to take their total to 50, which would give them control of the Senate – assuming Hillary Clinton becomes President and her Vice President, Tim Kaine, holds the tie-breaking vote as President of the Senate.

Nine of this year’s seats are held by Democrats and look safe for them, leaving five more required. There are two Republican-held seats that the Democrats look almost certain to take: Wisconsin, where former Senator Russ Feingold leads incumbent Ron Johnson by six points; and Illinois, where Congresswoman and Iraq war veteran Tammy Duckworth leads incumbent Mark Kirk by 14.

Then there are the six very close states that will most likely determine who controls the Senate as of 3 January: Indiana, Missouri, Nevada, New Hampshire, North Carolina and Pennsylvania. If the Democrats win three of these, it should be enough to get them to 50 seats in total.

The Republicans had seemed almost certain to retain Indiana’s seat despite incumbent Dan Coats standing down. The one Democrat who could pose a threat – Evan Bayh, who’d held it from 1999 to 2011 – announced in June 2015 that he wouldn’t run, and Democratic nominee Baron Hill trailed Republican Todd Young by double digits.

Then, in July 2016, Bayh reversed his decision and entered the race, and Hill stepped aside for him. Polls initially gave Bayh a big lead, but Young has steadily closed the gap and now trails by only around two points.

In Missouri, Republican Senator Roy Blunt faces unexpectedly strong opposition from the state’s Democratic Secretary of State, Jason Kander. Kander is running on an anti-Washington message against Blunt, who served in the House of Representatives for 14 years before ascending to the Senate in 2011. The latest polling has the two candidates neck-and-neck.

Having held his seat in Nevada in 2010 largely thanks to Republicans nominating Tea Partier Sharron Angle, Senate minority leader Harry Reid is retiring this year. That makes Nevada the one really vulnerable Democrat-held seat this time round. Catherine Cortez Masto, who served as the state’s Attorney General from 2007 to 2011, is roughly tied in the polls with Republican Congressman Joe Heck.

New Hampshire Republican Kelly Ayotte won election to the Senate by a big 23-point margin in 2010. This year, though, she faces not only less favourable national circumstances but also much tougher competition, in the form of the state’s governor, Maggie Hassan. Again, the polls have the candidates level.

Clinton’s relative strength with non-white voters and college-educated whites has brought North Carolina into play at the presidential level – and it seems to have made the Senate seat competitive too. Still, this looks like the hardest reach for Democrats of the six close races: Deborah Ross trails two-term incumbent Richard Burr by about two points.

Finally, there’s Pennsylvania, where the Democratic nominee is Katie McGinty, who served as an environmental adviser to Al Gore and chaired President Clinton’s Council on Environmental Quality in the Nineties. She’s taking on Republican Senator Pat Toomey, who narrowly won the seat in 2010 when he beat Democrat Joe Sestak 51–49.

The candidates had been locked together in the polls throughout the campaign, but McGinty seems to have pulled ahead recently, opening up a three-point lead in the last couple of weeks. That makes Pennsylvania now the Democrats’ best opportunity of these six states.

Given so many close races, how likely are the Democrats to make it to 50 seats overall? FiveThirtyEight’s polls-plus model currently gives them a 72 per cent chance of getting at least that many, while the New York Times’ forecast gives them a 61 per cent chance.

In other words, it’s more likely than not, but not overwhelmingly so. It’s not just the presidential contest that’ll keep us glued to the telly on Tuesday night.

Jonathan Jones writes for the New Statesman on American politics.