Deirdre Shesgreen, Erin Kelly, Eliza Collins, Bart Sullivan, Maureen Groppe, Ledyard King, Craig Gilbert and Heidi Przybyla

USA TODAY

WASHINGTON — Buckle up for a dramatic, vicious, multibillion-dollar fight for control of the Senate next year, which may end with almost no changes to the chamber’s narrow partisan divide.

It’s not for lack of opportunity: 33 senators are up for re-election in 2018, including 23 Democrats, two independents who caucus with Democrats, and eight Republicans.

But only about a dozen of those contests will be competitive. And two competing crosscurrents of this election — an unpopular Republican in the White House versus an electoral map that puts Democrats at a major disadvantage — could cancel each other out.

Republicans are defending far fewer seats, and only two GOP senators are considered truly vulnerable: Sens. Jeff Flake of Arizona and Dean Heller of Nevada. Meanwhile, 10 Senate Democrats are up for-election in states that Trump won in 2016, and they all have targets plastered on their backs.

Blowing in the other direction? History. The party that controls the White House almost always loses seats in Congress during midterm elections. President Trump, whose public approval ratings are below 40%, is expected to be a particularly heavy drag on the GOP in 2018.

Democrats would need to pick up three seats to retake control of the Senate, which analysts say is highly unlikely. Republicans would need a net gain of eight seats to hit the coveted 60-vote threshold to break a filibuster, which political experts say is equally far-fetched.

Of course, the political climate could change dramatically between now and November 2018, resulting in major shifts in the Senate’s current 52-48 partisan balance. And there’s no question that both parties — and their special-interest allies — will mount a pricey, pitched battle to sway the outcome.

Here's a race-by-race look at the key contests we're watching, with ratings from Inside Elections, which provides nonpartisan analysis and research. "Tossup" indicates the race is dead even; "tilt" ratings indicate a slight edge for the specified party; and "lean" ratings indicate a definite edge, but still an uncertain outcome.

Arizona

Incumbent: Republican Jeff Flake

Inside Elections rating: Tilt Republican

2016 presidential election results: Trump: 49%, Clinton: 45%

Flake’s re-election bid has gained national attention because Trump is trying to defeat him.

Flake has been an unwavering critic of the president. The senator’s new book, Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle, decries Trump’s noise over substance, his embrace of alternative facts and his “volatile unpredictability.”

Trump has bitten back, denouncing Flake in a tweet as “toxic” and applauding Flake’s opponent in the GOP primary, former Arizona state senator Kelli Ward. She lost a primary challenge against Sen. John McCain last year.

Trump also said Flake was “weak on borders.”

Flake, who grew up on a cattle ranch working alongside Mexican immigrants, helped craft a bipartisan immigration bill that would have beefed up border security while providing a pathway to citizenship for many undocumented immigrants already living in the country. The Senate passed the bill in 2013, but it died in the House.

Several Democrats are considering whether to run for the seat, including Rep. Kyrsten Sinema, the first openly bisexual person to be elected to Congress.

Florida

Incumbent: Democrat Bill Nelson

Inside Elections rating: Tilt Democratic

2016 presidential election results: Trump: 49%, Clinton: 48%

Nelson’s re-election bid could be his toughest campaign yet.

His home state went for Trump last year. He’s expected to face term-limited GOP Gov. Rick Scott, whose deep pockets and rising popularity make him the most formidable opponent Nelson has faced. Polls also indicate lukewarm support at best among Sunshine State voters for the Affordable Care Act, a law Nelson has generally championed and Scott has vehemently opposed.

But Nelson, who turns 75 later this month, remains a slight favorite to retain his seat for a few reasons: 1) Trump’s approval ratings remain underwater; 2) While Scott’s poll numbers have risen recently, Nelson’s net favorability ratings continue to outpace the governor's; 3) Despite deep concerns about Obamacare’s rising premiums and shrinking insurance options, voters in Florida overwhelmingly panned GOP efforts in Congress to repeal and replace the law.

Hurricane Irma could play an important role in the campaign, as it’s given Scott an opportunity to demonstrate leadership during the crisis, which could boost his approval ratings.

Indiana

Incumbent: Democrat Joe Donnelly

Inside Elections rating: Tossup

2016 presidential election results: Trump: 57%, Clinton: 38%

After Donnelly won his underdog race for the Senate in 2012, a national political pundit called him one of that year’s luckiest candidates.

He ran a smart race but hugely benefited from long-time senator Richard Lugar’s surprising primary loss, followed by GOP nominee Richard Mourdock’s subsequent missteps.

Now, as the only Democrat elected statewide in a state that overwhelming backed Trump last year, he’s one of the most vulnerable senators facing the voters in 2018. Outside groups started running ads in Indiana at the beginning of 2017.

A crowded field of Republicans are vying for the opportunity to take him on. But Democrats are enjoying the early attacks between the top GOP candidates — Reps. Todd Rokita and Luke Messer — which have been unusually personal.

Republicans are united, however, in their argument that, despite Donnelly’s moderate record, he votes with fellow Democrats when it matters most. They have an opening to define Donnelly on their terms because there are many Hoosiers still getting to know the freshman senator, despite his frequent travels around the state.

Cultivating an image of a low-key, keep-your-head-down-and-work-hard lawmaker has meant avoiding the spotlight, particularly on controversial or highly partisan issues. Instead, the topics Donnelly has focused on include agriculture, national defense and veterans’ issues.

The spotlight on veterans, as well as Donnelly’s emphasis on workers and jobs, may help him with the blue-collar voters who flocked to Trump last year.

But while appealing to Trump voters — whom Donnelly says were “originally Joe Donnelly supporters” — he will also have to keep liberal Democrats excited about turning out to vote for him.

Missouri

Incumbent: Democrat Claire McCaskill

Inside Elections rating: Tossup

2016 presidential election results: Trump: 57%, Clinton: 38%

Missouri’s Senate race was center stage last week when Trump parachuted into the state and took aim at McCaskill, urging the conservative crowd in Springfield to vote McCaskill out if she didn’t support the GOP’s tax cut plan.

McCaskill didn’t jab back, a sign that she’s treading carefully when it comes to Trump. First elected in 2006 amid a Democratic wave, McCaskill portrays herself as a no-nonsense moderate willing to buck her party on tough issues, such as environmental regulations.

With this election cycle already underway, McCaskill spent nearly all of August holding town halls in rural GOP-leaning parts of the state, defending her support for the Affordable Care Act and touting her willingness to fix its flaws. Her travel schedule reflects the reality that Missouri has become an increasingly conservative state, with Republicans sweeping every statewide office in 2016.

The GOP was similarly salivating over the prospects of ousting McCaskill in 2012, but she won handily — after she tinkered in the GOP primary to boost then-congressman Todd Akin. An arch conservative, Akin ignited a general election firestorm when he claimed that women who are victims of “legitimate rape” rarely get pregnant because their bodies have a way to “shut that whole thing down.”

This time, Republicans are laser-focused on finding a gaffe-proof recruit to challenge McCaskill, although their first choice — GOP Rep. Ann Wagner — threw party leaders a curveball earlier this year when she announced she wouldn’t run for the Senate. Republicans are now banking on Missouri’s attorney general, Josh Hawley, to unseat McCaskill.

Considered a rising GOP star, Hawley has tiptoed into the race amid pressure from GOP heavyweights, including Vice President Pence. A handful of other Republicans are also weighing a bid, setting up a potentially crowded primary.

Montana

Incumbent: Democrat Jon Tester

Inside Elections rating: Tilt Democratic

2016 presidential election results: Trump: 56%, Clinton: 36%

Tester could have faced the state’s sole Republican congressman, Ryan Zinke, next year had Trump not named Zinke secretary of the Interior, removing a formidable competitor from the field.

But the two-term Democrat with the distinctive flat-top haircut isn’t out of the woods. Montana voted overwhelmingly for Trump in 2016 and just elected Republican Greg Gianforte to replace Zinke in Congress, even after Gianforte physically assaulted a reporter.

The state is not toxic for Democrats. Months before the special congressional election this year, Montanans re-elected Democratic Gov. Steve Bullock. Tester narrowly beat incumbent Conrad Burns in 2006 and edged out his 2012 opponent, then-congressman Denny Rehberg, by 4 points.

State Auditor Matt Rosendale is the best known of three Republicans vying for the party’s nomination next year, and he sports the same buzzcut hairdo as Tester and many of the same "Make America Great Again" themes as Trump.

It helps Tester that he has remained scandal-free and well-funded. Tester has $4.7 million in the bank for a race sure to attract a lot of outside money.

Nevada

Incumbent: Republican Dean Heller

Inside Elections rating: Tossup

2016 presidential election results: Trump: 46%, Clinton: 48%

Nevada’s senior senator won his last election by just 1 point, and his state is becoming less red. It was one of the only battleground states Hillary Clinton won in the 2016 presidential race, when Nevada voters also elected the nation’s first Latina senator.

Heller already has at least one Democratic challenger, Rep. Jacky Rosen, who was able to flip Nevada’s 3rd District to blue in 2016.

But before Heller faces off against Rosen or another Democrat, he must first beat back a GOP primary challenge from Danny Tarkanian — a conservative who is closely aligned with Trump and has won multiple GOP primaries, though no general elections.

Heller has struggled with finding a clear message in his campaign so far, as he tries to convince Nevada’s conservative Republican base to vote for him in the primary and the more moderate electorate to also pick him in the general.

He slammed the GOP-crafted bill to repeal Obamacare before supporting a slightly narrower version. He also has voted to stop Medicaid reimbursements from going to Planned Parenthood, even though he promised to protect the women’s health care provider when asked about that issue during a town hall meeting.

Heller’s muddled messages have left both conservatives and Democrats confident they can beat him.

North Dakota

Incumbent: Democrat Heidi Heitkamp

Inside Elections rating: Tossup

2016 presidential election results: Trump: 63%, Clinton: 27%

Shortly after Trump handily won her state, Heitkamp met with him at his Trump Tower offices amid speculation she was being considered for a Cabinet post. She ended up without a nomination and voted against five of Trump’s Cabinet picks, but she broke with her party to support the president's Supreme Court nominee Neil Gorsuch.

To win re-election, Heitkamp will need to continue to distinguish herself from the Democrats' national brand, as she did supporting the Keystone XL pipeline, and hope that Trump’s national approval rating stays low.

Republican state Sen. Tom Campbell has announced he will challenge Heitkamp, perhaps heading off a bid by GOP Rep. Kevin Cramer, who had said he was considering a run.

Cramer worries some Republicans because of his penchant for making controversial, unscripted statements. Campbell's campaign is making a direct appeal to Trump voters. "I'll team up with President Trump to push his pro-business agenda," he posted on his Facebook page in August.

By creating an image as a centrist who can work with Trump, Heitkamp may offend progressives, but they’ll vote for her anyway. And she may be able to peel away Republicans and women who are concerned about Trump's unpredictability.

Heitkamp herself appears to be in no hurry to engage in the debate. She has not announced that she is running for re-election, her campaign website has not been updated since 2015, and she told Politico in June, "I haven't made up my mind" whether to run for another term. Nevertheless, she is paying for campaign staff and advertising, and she ended June with more than $3 million in her campaign coffers, so ...

Ohio

Incumbent: Democrat Sherrod Brown

Inside Elections rating: Lean Democratic

2016 presidential election results: Trump: 52%, Clinton: 44%

Ohio’s Senate race is likely to be a sequel — a political blockbuster pitting Brown against Republican state Treasurer Josh Mandel.

The two men ran against each other in 2012, with Brown defeating Mandel by about 6 points in one of that year's most expensive Senate contests.

The rematch will have at least one new plot twist. Mandel has a primary challenger — Mike Gibbons, a businessman and big-time GOP donor who has not run for public office before. Mandel is the frontrunner in the primary, although he has alienated some Ohio Republicans (including Gov. John Kasich) with his overtly ambitious persona.

Mandel has also shown a penchant for controversy, recently siding with a right-wing conspiracy theorist in a social media tiff with the Anti-Defamation League.

Brown has his own challenges, namely winning back Ohio’s blue-collar voters who abandoned the Democratic Party for Trump in 2018. Brown is also an unabashed liberal — in a state that is not. But Brown’s style of progressive politics, featuring crusades against free-trade deals and Wall Street excess, has helped him connect with working-class voters.

“Brown is endangered by the state’s strong swing to Trump last year,” Kyle Kondik, a political analyst with the University of Virginia’s Center for Politics, wrote in a recent analysis of Senate races. “However, the populist Brown is definitely a better fit for the Buckeye State than (Democratic presidential nominee Hillary) Clinton was.”

Pennsylvania

Incumbent: Democrat Bob Casey

Inside Elections rating: Lean Democratic

2016 presidential election results: Trump: 48%, Clinton: 47%

Casey is a moderate, generally soft-spoken Democrat from blue-collar Scranton who beat former GOP Sen. Rick Santorum nearly 12 years ago in part by touting his anti-abortion, pro-gun views.

But, instead of embracing Trump after his crucial upset victory over Clinton in Pennsylvania last year, Casey has blasted the president's Muslim travel restrictions, his deportation of immigrant families, and his failure to adequately condemn the white supremacist marchers who led the violent rally in Charlottesville, Va., last month.

Republicans say Casey has been moving too far left, an interesting charge leveled against a man whom Democratic activists have criticized for opposing abortion rights. The Irish Catholic senator and son of a former Pennsylvania governor has said he would like to overturn the landmark Supreme Court decision in Roe v. Wade, which legalized abortion.

Republican critics have questioned Casey's anti-abortion credentials, in part because he has opposed GOP efforts to defund Planned Parenthood, which performs abortions but is prohibited from using federal funds to do so. Casey argues that the group uses federal money to help prevent abortions by offering contraception services.

Republican Rep. Lou Barletta, a strong Trump supporter who was one of the first House members to endorse him in the presidential race, announced in late August that he'll challenge Casey. Several other Republicans also are expected to compete for the GOP nomination. Casey does not have an opponent in the Democratic primary.

Virginia

Incumbent: Democrat Tim Kaine

Inside Elections rating: Likely Democratic

2016 presidential election results: Trump: 44%, Clinton: 50%

Kaine may be the safest incumbent on this list. He is running in a state that voted for him as vice president on Hillary Clinton’s losing 2016 Democratic presidential ticket.

Thanks to rapid growth in the Northern Virginia suburbs of Washington, D.C., Virginia has maintained a deep bench of statewide Democratic elected officials, including its governor and two Democratic senators.

Corey Stewart, a strong supporter of Trump who in June narrowly lost the Republican gubernatorial primary to Ed Gillespie, has promised to launch a “vicious” campaign to unseat Kaine in 2018. Yet Democrats are assuming Trump’s approval ratings in the Commonwealth — lower than the national average — will hurt would-be Republican challengers, and Stewart has nowhere near Kaine's name recognition.

White nationalist violence during an August protest in Charlottesville could become a flashpoint in the race. Kaine said Trump showed “a complete lack of moral leadership” after Heather Heyer was killed by an alleged white supremacist who plowed his car into protesters.

Other Republicans who’ve said they are considering joining the race are former governor Jim Gilmore and retiring state Del. Jimmie Massie III.

West Virginia

Incumbent: Democrat Joe Manchin

Inside Elections rating: Tossup

2016 presidential election results: Trump: 68%; Clinton: 26%

Trump’s winning margin in Manchin's home state is far higher than nearly all of his Senate colleagues, including Republicans. And in a sign of Trump’s popularity in the state, West Virginia’s Democratic Gov. Jim Justice recently announced he was switching to the Republican Party so he could be more helpful to the president.

Manchin and Trump have had a good personal relationship and were speaking regularly earlier this year. But they grew distant after Trump asked the moderate Democrat to support Republican-led efforts to repeal and replace Obamacare, and Manchin said he could not.

Manchin’s spokesman told USA TODAY the two haven’t spoken for at least two months. According to the politics website FiveThirtyEight, this year Manchin has voted for legislation Trump supports about 54% of the time.

Rep. Evan Jenkins — a Democrat-turned-Republican who took West Virginia’s 3rd District from Democratic hands in 2014 — has announced he’s challenging Manchin. State Attorney General Patrick Morrisey is also in the running.

Manchin, a former governor who won his last race by 24 points, benefits from his name recognition in the state. A recent poll by West Virginia's MetroNews has Manchin ahead of Jenkins by 10 points and Morrisey by 14 points.

Wisconsin

Incumbent: Democratic Sen. Tammy Baldwin

Inside Elections rating: Tilt Democratic

2016 presidential election results: Trump: 47%, Clinton: 46%

Baldwin is half of the Senate’s leading political odd couple, one of the chamber's most liberal members paired with one of its most conservative (Republican Ron Johnson). That shows how Wisconsin has swung in recent years. Johnson got elected opposing Obamacare, while Baldwin is a vocal defender.

But Baldwin is roughly aligned with Trump on one set of issues — economic populism, including “fixing” trade deals and enacting “buy American” laws — that resonate with some voters in a state that’s more reliant on manufacturing than almost any other. One key to her campaign will be whether she can hold on to blue-collar counties she won in 2012 but swung from Obama to Trump in 2016.

Baldwin is the last big GOP target in a state where Republicans now dominate the legislature, hold five of eight U.S. House seats, have won the last three elections for governor and the most recent contests for U.S. Senate and president.

But she hasn’t yet drawn a big-name challenger, and GOP donors and activists have no consensus candidate. Republican Rep. Sean Duffy took a pass on the race. Announced or potential GOP challengers include state Sen. Leah Vukmir, wealthy businessman Eric Hovde and U.S. Marine veteran Kevin Nicholson.