Since Bernardo Arévalo burst onto Guatemala’s political scene last year as an anticorruption crusader, he has faced an assassination plot, his party’s suspension and a barrage of legal attacks aimed at preventing him from taking office as president.

Now comes the hard part.

Mr. Arévalo’s inauguration on Sunday — six months after his presidential victory delivered a stunning rebuke to Guatemala’s conservative political establishment — will mark a sea change in Central America’s most populous country. His landslide election reflected broad support for his proposals to curb graft and revive a teetering democracy.

But as Mr. Arévalo prepares to govern, he must assert control while facing off against an alliance of conservative prosecutors, members of Congress and other political figures who have in recent years gutted Guatemala’s governing institutions.

“Arévalo has the most thankless job in Guatemala today because he arrives with exceptionally high expectations,” said Edgar Ortíz Romero, a constitutional law expert. “He’s been given a budget for a Toyota when people want a Ferrari.”