The conservative group Club for Growth has booked $1.5 million in advertising in the upcoming Indiana primary. The ad explicitly urges voters to support Sen. Ted Cruz as a means of blocking Donald Trump from winning the Republican nomination.

“If you don’t want Donald Trump to win, your choice comes down to this: math,” the ad states. “Only Ted Cruz can beat Donald Trump. John Kasich can’t do it. The math won’t work.”

“Indiana is facing a unique moment in history: the opportunity to stop Donald Trump,” David McIntosh, President of Club for Growth Action, said. “After success with our ads in Wisconsin, Iowa, Oklahoma, and Utah, CFG Action is about to blanket the Hoosier State with a simple message: To stop Trump, vote Cruz. There is now no state more important than Indiana for electing Cruz and keeping Trump from reaching 1,237.”

Before he became President of Club for Growth, McIntosh was a Congressman from Indiana.

With the ad, the Club is clearly trying to replay the campaign tactics that delivered a lopsided win in the Wisconsin primary for Ted Cruz. While the RealClearPolitics average of polls showed Cruz leading in the Badger State by four points, he won the state by more than 13 points. The big margin allowed Cruz to secure 36 of the state’s 42 delegates.

The big difference between the polls in Wisconsin and the final results is that a lot of support for Kasich moved to Cruz at the very end. Kasich was polling around 20 percent support in the poll average, but ended up with just 14 percent of the vote. That vote shifted to Cruz at the very end of the campaign.

Indiana is emerging as one of the most important states remaining on the primary calendar for groups opposed to Donald Trump. After Trump’s landslide win in New York, the GOP frontrunner is expected to rack up a series of wins next week when five Northeastern states go to the polls.

The primary calendar then moves back to the Midwest and Western States, where Ted Cruz has run competitively against Trump. Similar to Wisconsin, Indiana is the only state voting on May 3rd. It is the only contest on the calendar between the primaries of April 26th and May 10th.

With a winner-take-most prize of 57 delegates, Indiana offers a critical lifeline for the Cruz campaign and its strategy of blocking Trump from collecting the 1,237 minimum he needs to secure a majority of pledged delegates. It Trump were to win Indiana, he would have a high probability of winning enough delegates to win the GOP nomination on the first ballot.

Groups opposed to Trump’s candidacy, like the Club, are banking on Indiana to blunt the frontrunner’s momentum going into the final primary states. As it stands now, Trump needs to win just under 60 percent of the remaining pledged delegates to secure the nomination. That challenge is higher if Trump were to lose Indiana.

The most recent poll of Indiana shows a very tight race between Trump and Cruz. The toplines of the poll show Trump with 37 percent against 31 percent for Cruz. Deeper internals of the poll, however, show the race to be essentially tied between the two. Ohio Gov. John Kasich is currently pulling 22 percent support from Indiana Republicans.

To replicate his win in Wisconsin, Cruz needs to capture a large share of the vote currently behind Kasich. The ad by Club for Growth is overtly making that appeal.

The primary in Indiana is critical. If Trump wins the state, it is very likely he will finish strongly in the remaining states and secure the necessary number of delegates to win the nomination outright. If, however, Cruz can capture the state, the primary will remain intensely competitive through June 7th, the final day of voting.

A loss in Indiana would also make it unlikely that Trump can win the 1,237 delegates he needs to secure the nomination before the RNC convention in July. The result could set in motion a scramble for delegates that will play out through the remaining months and the floor of the convention in Cleveland.

It is perhaps fitting that the GOP nomination comes down to a jump ball in Indiana.