By this time of the presidential primary season, President Obama had expected to step forward to unify the Democratic Party behind Hillary Clinton and against her Republican adversary.

At least, that was the plan.

But Vermont Sen. Bernie Sanders' surge in primary wins and fund raising has instead pushed the president to the sidelines -- and the self-described socialist senator couldn't be happier.

Despite pressure to call it quits, Sanders continues to be an undisputed fundraising machine. In February, he raised $43.5 million, according to filings to the Federal Election Commission – more than any other candidate in either party.

And as the money continues to flow, Sanders is showing no signs of slowing down, pledging instead to stay in the race through the July Democratic convention.

If Sanders wins Wisconsin’s primary on Tuesday – as polls seem to indicate – the White House will once again have to recalibrate its strategy, delaying key unity rallies the Democratic Party had banked on to bring together its base.

While Sanders remains a force in the Democratic primary, a win in Wisconsin would do little to significantly cut into Clinton's lead in delegates that will decide the party's nomination.

The stakes are higher on the Republican side.

Most polls show Texas Sen. Ted Cruz leading  front-runner Donald Trump in the Badger State. Wisconsin is a crucial state in his effort to push the party toward a convention fight.

"We are seeing victory after victory after victory in the grassroots," Cruz said during a campaign stop Monday. "What we are seeing in Wisconsin is the unity of the Republican Party manifesting."

Losses for Trump and Clinton in Wisconsin could be problematic with the next big contest on the primary calendar, in delegate-rich New York, not until April 19. Trump still has a comfortable lead in the Empire State, but Sanders has threatened to close the gap against Clinton on the Democratic side.

For Trump, the long lead-up to Wisconsin's contest has included one of the worst stretches of his candidacy. He was embroiled in a spat involving 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 in comments about abortion.

While 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 the GOP gathers in Cleveland.

Cruz headed into Tuesday's contest with the backing of much of the state's Republican leaders, including Gov. Scott Walker, but 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's still picking up delegates that would otherwise help Trump inch closer to the nomination or help Cruz catch up.

Trump has grown increasingly frustrated with the governor and has joined Cruz in calling for Kasich to end his campaign. Kasich cast Trump's focus on him as a sign that he's best positioned to win over the businessman's supporters.

"They're not really his people," Kasich said. "They're Americans who are worried about, they're really most worried about their kids, are their kids going to have a good life?"

If Cruz wins all of Wisconsin's 42 delegates, Trump would need to win 57 percent of those remaining to clinch the GOP nomination before the convention. So far, Trump has won 48 percent of the delegates awarded.

To win a prolonged convention fight, a candidate would need support from the individuals selected as delegates. The prolonged process of selecting those delegates would test the mettle of Trump's slim campaign operation.

Cruz prevailed in an early organizational 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 has so far endorsed Trump.

Among Democrats, Clinton has 1,243 delegates to Sanders' 980 based on primaries and caucuses. When including superdelegates, or party officials who can back any candidate, Clinton holds an even wider lead -- 1,712 to Sanders' 1,011. It takes 2,383 delegates to win the Democratic nomination.

Sanders would need to win 67 percent of the remaining delegates and uncommitted superdelegates to catch up to Clinton. So far, he's only winning 37 percent.

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 Associated Press contributed to this report.