The Never Trump forces at the Cleveland convention are likely to force a “freedom of conscience” floor vote, an attempt to free pledged delegates to vote for non-Trump candidates. The outcome of that vote—which will almost certainly fail—will make clear who controls the convention.

It’s precisely this sort of test vote that has made and unmade candidates in previous Republican conventions. In 1912, the Republican convention was a showdown between former president Theodore Roosevelt, who had won all but one primary, and sitting president William Howard Taft. The test vote was focused on the party’s temporary chairman. When Elihu Root, a Taft man, won, it became clear that Roosevelt’s forces would not prevail. Knowing he would lose, Roosevelt bolted the party. He became the Progressive Party nominee, and a split Republican Party cleared the way for Woodrow Wilson to win the presidency.

A test vote at the 1952 GOP convention led to a similar party shake-up. Robert Taft, Mr. Republican and son of William Howard Taft, was favored to win. But newcomer Dwight Eisenhower had emerged as a grassroots favorite. The problem? Ike had just joined the Republican Party and had never held elected office. So no one knew if he was a viable candidate.

When Eisenhower forces won a test vote on contested delegates, it became clear that he had the support he needed to win the nomination. As the New York Times reported at the time, “This started a bandwagon rolling for General Eisenhower and stopped Senator Taft, who had been the favorite before this convention met.”

The Never Trump forces will most likely fail in their test vote. But by using it to measure their strength at the convention, they are using a time-honored strategy for reshaping convention outcomes.