With polls showing him sliding nationally, Donald J. Trump received a bit of welcome news on Sunday in one battleground state as the editorial page of Nevada’s largest newspaper, The Las Vegas Review-Journal, endorsed him for president.

It is the first major newspaper to give Mr. Trump its blessing, though it may come with something of an asterisk: The Review-Journal was bought late last year by the casino magnate and billionaire Sheldon Adelson, a Trump supporter and longtime Republican benefactor.

The editorial described Mr. Trump as flawed but nonetheless preferable to Hillary Clinton.

“Mr. Trump’s impulsiveness and overheated rhetoric alienate many voters,” its endorsement said. “He has trouble dealing with critics and would be wise to discover the power of humility.”