Russian hackers would not be able to change the outcome of the United States presidential election, the nation’s most senior intelligence and law enforcement officials have assured Congress and the White House in recent weeks. But disrupting it, they acknowledge, would be far easier — causing doubts in battleground states, prompting challenges to results and creating enough chaos to make Florida’s hanging chads seem like a quaint problem from the analog age. By some measures, in fact, the disruption has already begun. And meddling around the edges of an election could sow doubts about the legitimacy of the results — especially in a year in which the Republican nominee, Donald J. Trump, has told his supporters that the only way he will lose is if the election is “rigged,” and while campaign officials for his Democratic opponent, Hillary Clinton, have held a series of meetings about preparing for the possibility that the vote will be hacked. The White House has declined to name Russia publicly as the chief suspect in a series of recent hacks, and has worded its public warnings carefully. The greatest danger, Lisa O. Monaco, President Obama’s domestic security adviser, said on Wednesday, is from attempts to cause “concern or confusion” about the voting system.