Election Day caps a tumultuous campaign season during which anger at the failures of the political system exposed extraordinary divisions within each party.

In Michigan last weekend, an astonishing Democratic rift emerged when the centrist Rep. Haley Stevens, who had narrowly lost the Senate primary, endorsed the Republican in the race and accused her party’s nominee, Abdul El-Sayed, of infusing his progressive agenda with antisemitism.

Republicans, in turn, have watched as President Trump elevated Ken Paxton, the indicted and impeached attorney general in Texas, for a Senate seat over a politically safer option, only to mock Paxton continuously on the trail as his campaign struggled. The seat is now in play, along with those in a handful of other states Trump won by double digits just two years ago, including Ohio, Iowa, Alaska and Kansas. One reason is that Trump angered many of his own voters by imposing tariffs and waging war with Iran, leading to higher prices.

The signs of the parties fracturing right before voters’ eyes came early, when Graham Platner, an obscure and unvetted oysterman from Maine, surged past the pick of party leaders with an angry, populist battle cry. “People know that the system is screwing them,” he said in a video launching his Senate bid. “They know it in their bones.”

Less than a year later, his meteoric campaign imploded, and he was gone.

The unruly nature of this year’s contests contains an appealing upside: the prospect that America’s unhappiness with the parties has grown so deep that it creates new possibilities for a political change, one that a swath of voters could find more satisfying.

Many voters this year have been re-evaluating their allegiance to either political party, creating more fluidity in the electorate. Voters have elevated a new generation of political leaders who have the potential to reshape the parties.

And voter dissatisfaction has produced surprising areas of agreement across party lines on how government can help people handle the cost of living. All three open the possibility that the parties have incentives to find new ways to become more responsive to voters. Tuesday’s elections will offer clues to just how much change can come from a new Congress and what likely will be the most wide-open presidential primary elections in years.“It does feel like we’re at an inflection point,” said Bill Haslam, a Republican former governor of Tennessee.

“The question is, is something now going to come out of it?”The optimistic view of the public’s frustration, he said, is that “the ‘pox on both your houses,’ might lead to ‘let’s try something new.’”

Voters reject partiesAmericans have been unhappy with their political leaders for years. In nine of the past 10 elections, voters have ejected one party or the other from its hold on the House, the Senate or the White House. The weight of history alone suggests that Republicans, as the party in power, have the tougher challenge this year.

The GOP’s own campaign investments in recent weeks show them racing to preserve a House majority that is at high risk and that, more remotely, even its Senate majority could slip away.

The dissatisfaction runs even deeper than in the past. Gallup last month found 15% of voters saying that most members of Congress deserve re-election, a record low in their polls dating to 1994. When Mark Putnam, a Democratic pollster, holds focus groups with voters, the anger reaches “an extremely poisonous level. One minute after they’ve introduced themselves, they start venting,” he said. “I’ve been doing this for 40 years, and I’ve never seen people as cynical and skeptical as they are now.”Increasingly, voters don’t see the parties as representing them.

Gallup last year found a record-high 45% identifying as independents, and recent Wall Street Journal polling found a big jump in the share of voters who hold unfavorable views of both parties. While Trump remains by far the dominant force in the GOP, fewer Republicans now identify with Trump’s MAGA movement, Journal polling shows, and a range of GOP candidates in recent weeks have broken with the president by disagreeing with his tariffs and the war with Iran.

Some party leaders are fretting that he took unnecessary risks this year in backing nominees such as Darline Graham, an inexperienced candidate who has put South Carolina in jeopardy of electing a Democrat to the Senate for the first time in 28 years.

Both parties have risked alienating voters with their tactics. Vivek Ramaswamy, the Republican candidate for governor in Ohio, ran an ad last month that was denounced as unseemly by the state’s governor and other GOP leaders for featuring his opponent’s estranged mother. In Alaska, the appearance of a candidate named Dan J. Sullivan Jr. on the ballot for Senate raised accusations that the Democratic candidate, Mary Peltola, was trying to siphon votes from her main opponent, incumbent Sen. Dan S. Sullivan.

Her campaign has denied the claim. Rising starsIn the wake of all this, the midterm results will show how elastic Americans are in their political affiliations—and what opportunity each party has to broaden its coalition. Meaningful shares of Latino voters, young voters, rural voters and some in the large bloc of working-class voters have told pollsters they may their dissatisfaction on Tuesday by staying home or switching their support from one party to the other.A new cohort of young political leaders is now introducing itself to this more fluid voter pool.

Sen. Jon Ossoff, a 39-year-old Democrat from Georgia, has drawn at least 3 million followers on social media with critiques of the Trump administration.

Democrat James Talarico, 37, is in the hunt for the Texas Senate seat due in large part to his oratory skills. Republican Rep. Brandon Gill, the youngest Republican member of the House, at 32, has about 2 million followers on Instagram and X. All are on the ballot on Tuesday.

Both parties are headed for internal fights over the best way to draw new voters. Republicans such as Vice President JD Vance are carrying the MAGA banner, while the non-MAGA wing has a voice in candidates such as John E. Sununu of New Hampshire, a Republican centrist running for the Senate, who says he is looking to shape a post-Trump America.“When Donald Trump leaves office in two years, where does the country go?” he asked in a debate.

Among Democrats, the roiling fight between progressives and centrists that defined the primary season presents the chance to build a bigger tent by harnessing the popularity of rising stars.“The parties are going to be realigning,” said Rep. Jake Auchincloss, a Massachusetts Democrat.

“And of course, it’s an opportunity for Democrats, but it’s also a massive risk”—a risk, he saw, of Democrats offering a watered-down version of Trump’s populism.

Auchincloss leads a group called Majority Democrats that is trying to steer the party into the political space he sees between Trump’s movement and the “activist left,” which he criticizes for holding ideological purity tests. “The only way to build wealth for the middle class is through the market economy,” he said, which might mean challenging zoning, pharmaceutical or other interests. Talarico is a member, as are other lawmakers. The goal of the Democratic Party, Auchincloss said, should be to draw in “the exhausted majority of Americans who want a party that is working for the middle class.”

The question is whether either party will try to build a durable majority by moving to the political center.

The impediments are high, said Ruy Teixeira, a political demographer at the American Enterprise Institute, a right-leaning think tank. Primary voters, who pick the nominees, tend to be more ideologically driven, not centrist.“It would take a real political entrepreneur to do it,” Teixeira said, “and sell it to voters. But I think the opportunity is real.”Write to Aaron Zitner at aaron.zitner@wsj.com