In November, 35 Senate seats are up for grabs as Democrats hope to take back a majority in Congress.

With American voters preparing to head to the ballot box on November 6, Democrats see key opportunities, in several states, to take back the majority in the US Senate and House of Representatives.

All 435 seats in the House of Representatives will be decided during the midterm elections, but only 35 of 100 Senate seats are up for grabs. The Democrats’ chances in the House appear to be much better than in the Senate, where several key races will be an uphill battle for the party.

Each of the 50 US states has two Senators in Congress. Senators serve staggered six-year terms, and for bills to become laws, they must pass through both chambers of Congress, the Senate and the House of Representatives.

For Democrats, taking back the Senate would allow them to block Trump’s Supreme Court nominations, disrupt his immigration agenda and stymy efforts to dismantle the Obama-era Affordable Care Act.

But Republicans currently hold a 51-49 majority in the Senate, and Democrats’ prospects for winning a majority are slim.

Here are some of the races you need to watch:

Arizona: Kyrsten Sinema v Martha McSally

Situated on the US-Mexico border, Arizona is witnessing a political battle brewing around immigration. The state is home to a large percentage of Latinos, some 31 percent of its total population, and Republicans have doubled down in their support of President Trump’s broad crackdown on immigration.

Republican Jeff Flake announced he would not seek re-election last year, leaving his seat open. For Republicans, the opportunity emerged to put a pro-Trump candidate in his place, and for Democrats, the opportunity to take the seat.

Martha McSally won the Republican primary after a heated election, and Republicans hope she can win the open Senate seat left behind. But Democratic challenger Kyrsten Sinema was leading by at least three points in a number of recent polls, with other opinion polls putting the pair neck and neck.

US Senate candidates: Martha McSally debates Kyrsten Sinema [Matt York/AP Photo]

Prediction website, FiveThirtyEight, says the race is currently leaning Democrat, with Sinema having a 61.1 percent chance of winning.

The Arizona race could have far-reaching consequences for the future of immigration, gun control and healthcare, among other key issues in the upcoming midterms.

Texas: Ted Cruz v Beto O’Rourke

In Texas, a traditionally Republican state, three-time congressman Beto O’Rourke has captured national attention by challenging incumbent Republican Senator Ted Cruz.

While most polls put Cruz ahead of O’Rourke by at least three percentage points, according to FiveThirtyEight, a Quinnipiac University survey found that 97 percent of black voters favour O’Rourke.

Beto O’Rourke debates Ted Cruz on October 16 [Tom Reel/San Antonio Express-News/AP Photo]

The second-most populous state in the country, Texas has not seen a Democrat win a statewide vote since 1994. But O’Rourke has so far bested Cruz in fundraising by a longshot, pulling in more than $24m in donations.

And, like in many states, immigration has taken a central role in their campaigns, with O’Rourke proclaiming support for a pathway to citizenship for undocumented immigrants and Cruz reiterating his support for Trump’s border wall.

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Although Cruz and Trump were at odds during the Republican primaries that eventually saw the latter win, Cruz has repositioned himself in the Trump camp. And Trump has thrown his weight behind Cruz, saying on Twitter: “Ted has my complete and total Endorsement. His opponent is a disaster for Texas.”

Nevada: Dean Heller v Jacky Rosen

For Democrats to break the Republican majority in the Senate, they need to win Nevada, where incumbent Republican Senator Dean Heller is facing off against Democratic contender Jacky Rosen.

During the 2016 presidential elections, Democratic candidate Hillary Clinton bested Trump in Nevada, where Democrats enjoy 60,000 more registered voters than their Republican counterparts. With a swelling Latino population, Rosen hopes to flip Nevada in the Democrats’ favour.

Formerly the president of her synagogue and a first-term congresswoman, Rosen has focused on immigration and healthcare.

Heller was also an outspoken supporter of newly minted Supreme Court Justice Brett Kavanaugh, whose confirmation hearings were marred by controversy over sexual assault allegations against the judge, which he denied. At one point he called the controversy a “hiccup”.

Rosen accused Heller of ignoring the voices of women in Nevada. Rosen has advocated for abortion rights, stricter gun control measures and the DREAM Act, which would have provided a path to citizenship for undocumented immigrants who came to the US as children.

FiveThirtyEight, as well as other projection sites, consider this race a toss-up but Heller has a very small lead in most polls.

Tennessee: Marsha Blackburn v Phil Bredesen

Tennessee may be a long shot for Democrats, but their potential victory in the solidly Republican state is crucial for regaining a majority in the Senate.

With Tennessee going to Trump by 26 points in 2016 and the president enjoying a 54 percent approval rating in the state, the Democrats face an uphill battle with former Democratic Governor Phil Bredesen’s bid to take the vacant seat left behind by Bob Corker, a Republican Trump critic who recently announced his retirement.

Phil Bredesen debates Marsha Blackburn on October 10 [Mark Humphrey/AP Photo]

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Blackburn has aligned herself with Trump’s brand of Republicanism, rallying against the Obama-era Affordable Care Act and accusing Bredesen of supporting single-payer healthcare. As governor, Bredesen introduced public spending cuts that resulted in some 200,000 Tennesseans losing their spots on TennCare, the state Medicaid programme.

Most recent polling puts Blackburn ahead by at least four percentage points. FiveThirtyEight says this race will likely go Republican, with the party having an 80.4 percent chance of winning

West Virginia: Joe Manchin v Patrick Morrisey

Although Trump won West Virginia by 40 points in 2016, incumbent Democrat Senator Joe Manchin remains popular in the state, enjoying a 46-percent support rating, according to a recent poll by Gray Television.

Meanwhile, Republican challenger State Attorney General Patrick Morrisey received 38 percent in the same poll.

But President Trump, who boasts of a 62 percent approval rating in West Virginia, has thrown his support behind Morrisey, holding several campaign rallies for the Republican hopeful. FiveThirtyEight predicts that Democrats have an 87.2 percent chance of retaining Manchin’s Senate seat, but an upset victory could spoil the party’s hopes of taking back the Senate.

Attempting to capitalise on his broad support in the state, Trump has described Morrisey as “spectacular” and turned West Virginia into a battleground over the Kavanaugh confirmation hearings even as Manchin became the only Democrat to throw his support behind the judge.

“We see this horrible, horrible, radical group of Democrats. You see what’s happening right now,” Trump told thousands of supporters at a recent rally in West Virginia.

“And they’re determined to take back power by any means necessary. You see the meanness, the nastiness. They don’t care who they hurt, who they have to run over to get power.”

Indiana: Joe Donnelly v Mike Braun

US Senator Joe Donnelly, who is facing off against Republican challenger Mike Braun in Indiana, is considered one of the most vulnerable Democrats in the midterm elections.

Indiana is one of five states Trump won by double digits in 2016, and outside groups have poured more than $45m into races in the state since May, according to local media reports.

Several recent polls have shown Braun leading Donnelly by more than three points.

With Braun accusing his opponent of being part of the Democratic Party establishment, Donnelly has attempted to fashion himself as an outlier who often votes with the right-wing of the Republican Party.

“The radical left wants to eliminate ICE,” stated a recent Donnelly ad. “I support ICE and funding President Trump’s border wall.”

Missouri: Claire McCaskill v Josh Hawley

In Missouri, where Trump won by 19 points two years ago, 65-year-old Democratic Senator Claire McCaskill has weathered controversial attack ads, secret filming as part of a right-wing sting operation and accusations of being a “DC insider”.

Her opponent, 38-year-old Josh Hawley, enjoys Trump’s backing and is neck-and-neck with McCaskill in the polls. The race has turned ugly in recent weeks.

Paid for by the right-wing Stars and Stripes Forever PAC, a pro-Hawley ad drew controversy for claiming that “Christians, Jews and Muslims” do not “worship the same god”.

The ad concluded, “Vote no on Claire McCaskill and her Democrat agenda. Vote yes for our children.”

In October, McCaskill’s campaign revealed that it was infiltrated by Project Veritas, a far-right NGO known for targeting liberals and leftists in undercover operations.

The videos showed a man grilling the Democratic incumbent on her positions on gun control, Trump’s plan to build a wall on the US-Mexico border and other hot-button issues.

FiveThirtyEight puts McCaskill’s chances of besting Hawley at four in seven.

North Dakota: Heidi Heitkamp v Kevin Cramer

In North Dakota, Democratic incumbent Senator Heidi Heitkamp has been losing ground to Republican Congressman Kevin Cramer.

Cramer has railed against Heitkamp’s vote against the confirmation of controversial Supreme Court Justice Brett Kavanaugh and her opposition to Trump in a state where the Republican president won handily in 2016.

“You vote against Brett Kavanaugh when 64 percent of your constituents want you to vote for Brett Kavanaugh?” Cramer asked rhetorically in a debate recently.

The two have barbed over the Obama-era Affordable Care Act, which Cramer wants to axe and Heitkamp supports, as well as Trump’s recent introduction of tariffs on imported Chinese goods.

China’s reply – also billions of dollars of tariffs on US products – hit North Dakota soybean farmers hard, according to local media reports.

FiveThirtyEight predicts that Cramer has a nearly 74.9 percent chance of ousting Heitkamp.