– Advertisement – Presidential elections always provoke anxiety, but this year’s campaign is closing on an especially unnerving note, with reports of pre-election vandalism, the boarding-up of stores in anticipation of rioting and the specter of voter intimidation.On Monday morning, officials arriving at the Democratic headquarters in Harris County, Tex., found the locks on the front door sealed with glue, and slogans and blobs of red paint smeared on windows.- Advertisement – – Advertisement – On Monday, the F.B.I. confirmed that its San Antonio office was investigating an incident in which a caravan of Trump supporters surrounded a Biden campaign bus on Friday — an act of intimidation that President Trump praised on Twitter.In New York City, the windows of Saks Fifth Avenue, which have wowed tourists for decades, were boarded up on Monday morning. SoHo, where trendy shoppers once flocked to glittering stores, echoed with the sound of hammers. On the sidewalk outside the Disney Store in Times Square, workers attached plywood to the storefront.The sea of plywood stretched into more modest commercial districts in the Bronx and Brooklyn, and even to Washington, where in recent days the ominous precaution has been evident all across downtown, fanning out several blocks from the White House, spreading around Capitol Hill, transforming the nightlife corridors of 14th Street and Adams-Morgan and reaching up into the suburbs. – Advertisement – The preparations reflected a broader national anxiety surrounding the contest between Mr. Trump and Joseph R. Biden Jr., and the growing fears that no matter who wins, the aftermath of the election could include violence.The weekend saw tensions flare up. In North Carolina on Saturday, the police used a chemical spray to disperse a get-out-the-vote rally. On Sunday, cars and trucks with Trump flags halted traffic on the Garden State Parkway in New Jersey and jammed the Mario M. Cuomo Bridge in New York’s northern suburbs, and a pro-Trump convoy in Virginia ended in a tense shouting match with protesters as it approached a statue of Robert E. Lee in Richmond.States are already on alert. On Monday, Gov. Charlie Baker of Massachusetts ordered 1,000 members of the National Guard to be on standby. In Oregon, which has seen months of sporadic unrest, Gov. Kate Brown ordered the state National Guard to remain on standby in case violent protests erupt.“We know that there are some people who might use peaceful election night protests to promote violence and property destruction,” Ms. Brown said Monday. “That behavior is not acceptable.” On Sunday, students at George Washington University received an email headlined “We Suggest Preparing for the Election Day Period as you Would for a Hurricane or a Snowstorm.” Reporters at The Wall Street Journal’s Washington bureau were issued gas masks and orange bike helmets marked “press.” It appeared that nearly every CVS, Walgreens and 7-Eleven within at least a mile or so of downtown was being heavily fortified. In the suburbs of Pittsburgh, aides to Representative Conor Lamb, a Democrat, arrived at their storefront office to a similar scene: It was defaced with a red hammer-and-sickle sign and the words, “Don’t vote! Fight for revolution.”Police said they were investigating both incidents but had not yet identified any perpetrators.Throughout the country, business owners and government officials — from the managers of Saks Fifth Avenue to the president’s staff — were bracing for potential acts of vandalism or violence based on the outcome, or lack of an outcome.