MEXICO CITY — Winning Mexico’s presidency this summer did not slow down Andrés Manuel López Obrador: Three months into his transition, he is traveling across the country to thank voters, replay his campaign promises and pledge, “I will not fail you.”

But now there’s often an asterisk: “We wish we could give more.”

Mr. López Obrador was elected by a landslide in July on a mandate to battle corruption, reduce soaring violence and tackle the country’s entrenched inequality. Now those promises are colliding against a reality too complex to be reduced to a stump speech.

As a result, a new note has crept into his speeches. He has backtracked on many of his signature issues and hedged on his commitments, trying to whittle down his supporters’ outsize expectations.

At stops around the country, he is renewing his pledges to provide cash grants for young people, higher pensions for retirees, price supports for farmers and loans for small business. Only now he adds, “We are not going to spend more than what comes in,” as he told a rally recently.