The key US Senate races that could determine who controls the chamber

The Democrats are fending off a Republican offensive aimed at overturning their tiny majority in the Senate.

Retaking control of the US Congress’s upper chamber may represent the GOP’s best opportunity of success in November’s election, according to analysts, surpassing their chances of retaking the White House or even retaining control of the House of Representatives.

But with a burst of enthusiasm from Kamala Harris’s campaign, Democrats are still competitive.

What is the Senate’s current makeup?

The Democrats currently hold 48 of the 100-member chamber’s seats plus having the support of three independent senators who sit with the party’s caucus and back it on important votes. The Republicans have 49 seats, meaning they only need to gain two in November – when one-third of the seats are up for grabs – to regain control.

How likely are Republicans to take control?

On the face of it, the GOP have the edge. The retirement of Joe Manchin, a centrist Democrat from West Virginia with a track record of taking Republican-friendly positions, is almost certain to result in a GOP gain in a state that has become a party stronghold.

If Donald Trump were to retake the White House, winning West Virginia alone would be enough to in effect hand the Senate to the Republicans, with the vice-president being awarded the casting vote under the US constitution in a 50-50 split scenario.

Where are sitting Democrats most vulnerable?

Two Democratic incumbent senators in GOP heartland states are seen as particularly vulnerable; Jon Tester of Montana and Sherrod Brown of Ohio. The latter is a one-time election battleground that has been won by Trump in recent elections and is now considered solid Republican territory.

In states that have become increasingly conservative, both Tester and Brown have sought to stay competitive by emphasising their independence from the more liberal national party organisation. Each stayed away from August’s Democratic national convention in Chicago to focus on wooing independent and swing voters whose support is deemed vital to holding their seats. The Cook Political Report has designated Montana as leaning Republican and Ohio as a toss-up.

Montana
John Tester v Tim Sheehy

Tester, who has held the seat since 2006, is running for a fourth term in a state Trump won by 16 points four years ago against Tim Sheehy, a former Navy Seal and businessman, who has the former president’s backing.

Read David Smith’s reporting about the Montana Senate race here.

Ohio
Sherrod Brown v Bernie Moreno

Brown has been in the Senate for as long as Tester and is also seeking his fourth term. His opponent, Bernie Moreno, is another Trump-backed candidate who was not the choice of the Republican establishment in Ohio.

Read Stephen Starr’s reporting on the Ohio Senate race here.

Key presidential battleground states where the Republicans hope to make gains

Nevada
Jacky Rosen v Sam Brown

Jacky Rosen, a first-term senator elected in 2018, is facing a challenge from Sam Brown, a Republican with a bracing back story but a thin political résumé. Brown, a graduate of West Point military academy and veteran of the long US-led military effort in Afghanistan, barely survived a roadside bomb attack in Kandahar that left him permanently scarred. Like Tester and Brown, Rosen opted to skip the recent Democratic convention to focus on her Senate defense.

The Cook Report categorizes the race as leaning Democrat.

Wisconsin
Tammy Baldwin v Eric Hovde

The two-term senator Tammy Baldwin is up against Eric Hovde, a wealthy GOP banker and businessman who is funding his own campaign. But his local bona fides have been questioned by Democrats who have stressed his connections to southern California.

The race is judged as a toss-up by the Cook Report.

Read Alice Herman’s reporting about the Wisconsin Senate race here.

Pennsylvania
Bob Casey v David McCormick

The three-term incumbent Democratic senator Bob Casey is pitted against David McCormick, former chief executive of Bridgewater Associates, one of the world’s largest hedge funds.

Casey is favoured to hang on to his seat, partly because of strong local name recognition. His opponent has already suffered defeat in a Pennsylvania Senate race, having narrowly lost the Republican state primary in 2022 to Mehmet Oz, who then lost to Democrat John Fetterman in the election for a seat that had been left vacant by the retirement of a GOP incumbent, Pat Toomey, one of only seven Senate Republicans to vote to convict Trump over the January 6 attack on the US Capitol at his second impeachment trial.

The Cook Report also classes it as a toss-up.

Read Joan E Greve’s reporting about the Pennsylvania Senate race here.

Arizona
Ruben Gallego v Kari Lake

A seat left vacant by the retirement of Kyrsten Sinema, a former Democrat who quit the party to become an independent, is now a key target of Kari Lake, a one-time television news anchor who has positioned herself as a vocal Trump supporter and arch-exponent of the lie that the 2020 election was stolen. She repeated the tactic when she lost the 2022 Arizona governor’s race to Democrat Katie Hobbs. Her opponent is Gallego, a Democratic member of the House and former Marine who served in Iraq.

The Cook Report judges the contest as leaning Democrat.

Read Lauren Gambino and Rachel Leingang’s reporting about the Arizona Senate race here.

Michigan
Elissa Slotkin v Mike Rogers

Here the Republicans are on the offensive to replace a retiring Democratic senator, Debbie Stabenow, who is stepping down after 24 years. Trying to fill her shoes will be Elissa Slotkin, currently a Democratic member of the House, who will be in a race with Mike Rogers, a mainstream Republican who once said his party had to move beyond Trump before changing tack to embrace him, earning the former president’s endorsement.

The race is considered a toss-up by the Cook Report.

Maryland
Angela Alsobrooks v Larry Hogan

Solidly Democratic Maryland, which includes the liberal suburbs of Washington DC, is crucial to the party’s hopes of clinging to its Senate majority.

Instead, it’s turning into a dogfight between the Democratic candidate, Angela Alsobrooks, the executive of one of Maryland’s biggest counties, and the state’s former Republican governor Larry Hogan. An American Association of Retired Persons poll showed the pair at 46% each – despite Harris holding a 20% state lead in the presidential race. Part of the explanation is the moderate public profile of Hogan, a traditional Republican who has positioned himself as anti-Trump and has said he does not seek the former president’s endorsement.

Indeed, buoyed by the recent poll, Hogan has now gone further, marketing himself in a fresh ad as one of the few Republicans who has never “caved” to Trump and extolling his decision to dispatch the Maryland national guard to put down the 6 January 2021 attack on the US Capitol.

The state that Trump lost to Joe Biden by 33 points in 2020 is in likely Democratic territory, according to the Cook Report.

Read Martin Pengelly’s reporting about the Maryland Senate race here.

Possible dark horse races?

Florida
Where the Republican incumbent, Rick Scott, is being challenged by the Democrat Debbie Mucarsel-Powell.

Texas
Colin Allred, another Democrat, is trying to unseat the combative sitting senator, Ted Cruz, who ran unsuccessfully for the Republican presidential nomination in 2016.

The Cook Report assesses the Florida race as likely Republican, and the Texas contest as leaning Republican.