There have been moments in the Republican campaign when Donald Trump seemed close to locking up the party’s presidential nomination. The candidate who won New Hampshire, South Carolina, Nevada, Michigan, Massachusetts, Georgia, Florida, Arizona, and more displayed strength across the country that would, in normal times at least, lead to overall victory. Yet today, despite his solid standing in New York and other upcoming northeastern primaries, and his continued dominance of national polls, Trump has a difficult and shaky path to the 1,237 delegates required to win the GOP nomination on the first ballot.

His prospects for the second and later ballots look iffy, as the Cruz campaign works with #NeverTrump operatives to peel away delegates who are required to vote for Trump initially but will be free to go elsewhere on later ballots. In short, Trump had better win the nomination on the first pass. If he doesn’t, the front-runner could well lose.

How did that happen? Talks with people knowledgeable about the Trump campaign point to two moments — one in February, one in March — when Trump and his aides made mistakes that would undermine all the work they had done to take them to the verge of the GOP nomination.