Republican Ted Cruz and Democrat Bernie Sanders are angling for victories in the Wisconsin presidential primaries that could give their campaigns a much-needed boost.
While Mr Sanders remains a force in the Democratic primary, a win over Hillary Clinton in Wisconsin would do little to significantly cut into her lead in delegates that will decide the party's nomination at the national convention in July.
The stakes are higher for Ted Cruz, the Texas senator, who trails Donald Trump in the Republican race and sees Wisconsin as a crucial state in his effort to push the party towards a contested convention.
"We are seeing victory after victory after victory in the grassroots," Mr Cruz said.
"What we are seeing in Wisconsin is the unity of the Republican Party manifesting."
With the White House and control of Congress at stake in November, leaders of both parties are eager to turn their attention toward the general election.
Mrs Clinton would enter the general election campaign saddled with persistent questions about her honesty and trustworthiness, but also with a significant demographic advantage.
It is an edge Democrats believe would be magnified in a race against Donald Trump, who has made controversial comments about Hispanic immigrants and women.
For Mr Trump, the long lead-up to Wisconsin's contest has included one of the worst stretches of his candidacy.
The billionaire real estate mogul was embroiled in a spat involving Mr Cruz's wife, which he now says he regrets, was sidetracked by his campaign manager's legal problems after an altercation with a female reporter, and stumbled awkwardly during comments about abortion.
While Mr Trump is the only Republican with a realistic path to clinching the nomination ahead of the Republican convention, a big loss in Wisconsin would greatly reduce his chances of reaching the 1,237 delegates needed to do so before Republicans gather in July for their convention in Cleveland.
During a rally in Milwaukee, Mr Trump brought his wife Melania on stage.
In a short, prepared speech, Mrs Trump defended her husband against critics who say he is alienating female voters and threatening his electability in a general election, especially in a battle against Democrat Hillary Clinton.
"No matter who you are, a man or a woman, he treats everyone equal," Melania Trump said.
She said her husband is strong, tough and a great leader, adding: "When you attack him, he will punch back 10 times harder."
Heading into Wisconsin, Trump has 737 delegates to Cruz's 475, with Ohio governor John Kasich trailing with 143.
Mr Cruz goes into Tuesday's contest with the backing of much of Wisconsin's Republican leaders, including Governor Scott Walker, but Mr Trump made a spirited final push in the state and predicted a "really, really big victory".
"If we do well here, it's over," he said. "If we don't win here, it's not over."
Complicating the primary landscape for both Cruz and Trump is the continuing candidacy of John Kasich. The Ohio governor's only victory has come in his home state, but he is still picking up delegates that would otherwise help Mr Trump inch closer to the nomination - or help Mr Cruz catch up.
Donald Trump has grown increasingly frustrated with the governor and has joined Ted Cruz in calling for Mr Kasich to end his campaign. Mr Kasich cast Mr Trump's focus on him as a sign that he is best positioned to win over the businessman's supporters.
"They're not really his people," Mr Kasich said.
"They're Americans who are ... mostly worried about their kids. Are their kids going to have a good life?"
If Ted Cruz wins all of Wisconsin's 42 delegates, Mr Trump would need to win 57% of those remaining to clinch the Republican nomination before the convention.
So far, Mr Trump has won 48% of the delegates awarded.
To win a prolonged convention fight, a candidate would need support from the individuals selected as delegates.
The process of selecting those delegates is tedious, and will test the mettle of Mr Trump's slim campaign operation.
Ted Cruz prevailed in an early organisational test in North Dakota, scooping up endorsements from delegates who were selected at the party's state convention over the weekend.
While all 28 of the state's delegates go to the national convention as free agents, 10 said in interviews that they were committed to Cruz. None have so far have endorsed Donald Trump.
Among the Democrats, Hillary Clinton has 1,243 delegates to Bernie Sanders' 980 based on primaries and caucuses.
When including superdelegates, or party officials who can back any candidate, Mrs Clinton holds an even wider lead - 1,712 to Sanders' 1,011. It takes 2,383 delegates to win the Democratic nomination.
Mr Sanders would need to win 67% of the remaining delegates and uncommitted superdelegates to catch up to Clinton. So far, he is only winning 37%.
Even if Sanders wins in Wisconsin, he's unlikely to gain much ground. Because Democrats award delegates proportionally, a narrow victory by either candidate on Tuesday would mean that both Sanders and Clinton would get a similar number of delegates.
The Democratic contest has taken a more contentious turn in recent weeks, as the candidates turn their focus to what is expected to be a drag-out fight ahead of the New York primary, a must-win state for Mrs Clinton.
She represented New York in the Senate, while Mr Sanders was born in Brooklyn. The two candidates have agreed to hold a debate on April 14 in Brooklyn.