Set aside the polls, the fundraising numbers, and President Trump’s name recognition as metrics of his early dominance of the Republican presidential contest. He has what could prove to be the most important advantage in the race: a leg up in winning the delegates needed to clinch the GOP nomination.

While the delegate count won’t begin taking shape until voting begins next January, Mr. Trump’s edge in the race to win their votes is years in the making. Many state Republican parties made changes to their rules ahead of the 2020 election by adding more winner-take-all contests and requiring candidates to earn higher percentages of the vote to claim any delegates.