Perhaps, perhaps not, but a win by Donald Trump would send a powerful message to voters in the remaining primary states — and to skeptical Republican leaders, too. Get on board now, the message will read, or be sorry later. In that sense, Karl Rove’s observation on Fox News last night is spot on:

“If he wins Wisconsin, the contest is over,” the former George W. Bush White House aide and Fox News contributor said. “If not, it’s gonna go on and the math becomes somewhat more difficult.”

Trump leads the GOP field but is still 500 delegates shy of clinching the nomination. Ted Cruz trails Trump by more than 260 delegates but holds a 5-point advantage heading into Wisconsin’s primary, according to a RealClearPolitics average of state polls. Rove, however, suggested Wisconsin’s open primary process favors the New York billionaire.

“Trump has done slightly better in states where there is an open primary, where Democrats and independents can come in and vote, and has done slightly worse in states that have a closed primary, where only registered Republicans can vote,” he said.

Wisconsin holds open primaries, as voters do not declare a party affiliation while registering. Like other open primary states, voters have to choose one party’s ballot when voting in a primary. How many Democrats will cross over into the GOP primary, though, with a close race between Bernie Sanders and Hillary Clinton on their own ballot? The RCP average puts Sanders up by less than three points, although Hillary has only led in three of the eight polls taken over the last month — and one of those (Emerson) put Bernie up eight in its final iteration.

The gold standard for Wisconsin polling, Marquette Law School, found that very few Democrats planned to cross over, although Republicans will get a +20 in self-professed independents:

Among likely voters in this new (March) poll, 54 percent say they will vote in the Republican primary while 46 percent choose the Democratic primary. Ninety-nine percent of Republicans say they will vote in the Republican primary with 1 percent choosing the Democratic contest. Among Democrats, 95 percent say they will vote in that party’s primary, with 5 percent crossing over to the GOP. Independents, including those leaning to either party, choose the Republican primary over the Democratic primary by 60 percent to 40 percent.

Unfortunately, Marquette does not break out the crosstabs for support among voting demographics, so Trump’s ability to surprise among independents can’t be measured easily from their data. However, Cruz won 53% of likely Milwaukee city and county voters to Trump’s 15%, scored 43/27 over Trump in the rest of the Milwaukee media market, and carries the critical Green Bay media market by 41/32. While Trump has done well generally in open primaries, it might not be as much of a factor in Wisconsin.

There’s also this:

Among the 80 percent of likely Republican primary voters who approve of the job Walker is doing as governor, Cruz has the support of 45 percent, Trump 27 percent and Kasich 18 percent, with 9 percent undecided. Among the 17 percent who disapprove of Walker’s job performance, 44 percent back Trump, 38 percent Kasich and 16 percent Cruz, with just 1 percent undecided.

Trump’s attack on Walker was a poorly chosen strategy, as I remind readers in my column at The Week:

Throughout the country, Trump resonates with disaffected Republicans in large part because of their frustration with unrealistic promises made by the GOP in the last two midterm elections — and the failure to deliver on these promises, especially on immigration and ObamaCare. Republican majorities in both the House and Senate failed to deliver either a repeal or defunding of ObamaCare despite myriad promises to force President Obama to cave on his signature accomplishment. The only progress on immigration came from the reviled Gang of Eight proposal that conservatives considered an amnesty program, and which sunk Marco Rubio’s presidential aspirations in 2016.

But in Wisconsin, Republicans have had a much different experience over the last five years. And that could be why Trump’s attacks on the establishment have not generated the same enthusiasm in Wisconsin that they have elsewhere.

Badger State voters elected Scott Walker as governor and a Republican-controlled legislature in 2010, and these conservative lawmakers wasted no time in making good on their promises. They passed reforms on public-employee unions to prevent a budget crisis and to undercut the political power of Big Labor in the state. That reform alone saved Wisconsin taxpayers over $5 billion in the years since. Walker and the GOP also passed a voter ID law that has held up in the courts, and passed right-to-work legislation. Walker has managed to pass balanced budgets without raising taxes, too, another issue on which the grassroots and so-called “establishment” see eye to eye. It was this track record of accomplishment that allowed Walker to be the first governor in U.S. history to withstand a recall election, easily beating back the attempt to remove him from office, and led to his re-election two years later.

Trump came into Wisconsin attacking Walker, mainly because Walker had repeatedly criticized Trump earlier in the campaign. (Walker has endorsed Cruz.) Strangely, Trump attacked Walker for not raising taxes and for not cooperating more with Democrats, arguments that left Wisconsin Republicans scratching their heads. The attacks backfired, and Cruz quickly rose in polling. Trump later reversed himself on Walker, but the episode highlighted a big problem in Trump’s campaign. Team Trump just assumed that his national anti-establishment message (and usual pattern of personal attacks) would work in Wisconsin. Trump and his team clearly didn’t bother to learn anything about the voters in these communities first.

It’s possible that Trump has rebounded somewhat since the Marquette poll, although the ARG poll is the only one indicating such a rebound, and it has significant problems. Trump may win some delegates tonight, but it looks more like the Republican primary race will remain wide open after tonight. Watch the exit polls for data on late deciders to get a sense of whether Trump managed a comeback in a state he didn’t understand very well.