Schools and government agencies in the Washington area began bracing Friday for Hurricane Sandy, as forecasters predicted that the potential collision of a hurricane and a nor’easter could pummel the region with relentless rains, winds and flooding for several days, starting as early as Sunday.

Meteorologists said the confluence of two such storm systems along a heavily populated corridor like the mid-Atlantic was extremely rare, if not unique.

“This is a beyond-strange situation. It’s unprecedented and bizarre,” the Weather Channel’s Brian Norcross posted on his Facebook page.

The Washington Post’s Capital Weather Gang warned that “this storm may be unlike anything the region has ever experienced.”

In a worst-case scenario, the monster clash of storm systems, dubbed “Frankenstorm” by some forecasters, could be the worst to hit the northeastern United States in a century, affecting 60 million people and causing billions of dollars in damage.

“Sandy will not be your typical hurricane,” AccuWeather reported. “Sandy will be more like a large nor’easter on steroids. It could have the strength of a Category 1 or 2 hurricane. Tropical storm and hurricane-force wind gusts will extend out hundreds of miles from the center.”

The private forecasting firm said the storm’s worst effects would come Monday through Tuesday and that damaging wind gusts would reach from Boston to Washington and inland to the central Appalachians.

Even as the East Coast braced for Sandy’s arrival, the storm began to affect the U.S. presidential race Friday. The campaign of Republican candidate Mitt Romney announced the cancellation of a planned Sunday night rally in Virginia Beach because of the hurricane.

Maryland Gov. Martin O’Malley (D) declared a state of emergency and warned that the onslaught could have an impact on early voting ahead of the Nov. 6 elections. Early voting in Maryland is scheduled to take place between Oct. 27 and Nov. 1.

For now, early voting in Maryland will continue this weekend, said Margie Roher, a spokeswoman for Montgomery County’s election board. O’Malley and the state elections board can jointly decide to change the early voting schedule, but that wouldn’t occur until the storm hits, Roher said.

President Obama received an update on Sandy from Federal Emergency Management Agency Administrator Craig Fugate, National Hurricane Center Director Rick Knabb and homeland security adviser John O. Brennan. Obama directed FEMA to ensure that all available federal resources are being brought to bear to support state and local responders, the White House said.

“At the request of states, FEMA is proactively deploying Incident Management Assistance Teams to multiple states up and down the Eastern Seaboard to assist efforts as state and local partners prepare for the storm,” a White House statement said.

Hurricane Sandy blasted the Bahamas on Friday after raging through the Caribbean, killing at least 29, wire services reported. Forecasters said it is too early to know whether the storm would pass directly over land in the mid-Atlantic or veer out to sea — and too soon to gauge how quickly it would arrive in the Washington region. But with the nor’easter expected to bear down at about the same time, most predictions were that the impact would be significant.

With the Marine Corps Marathon planned for Sunday, followed three days later by Halloween, the potential for major disruptions seemed significant.

A marathon spokeswoman, Tami Faram, said that “right now, we’re going on with all of our events as planned.” The race starts at 7:55 a.m. Sunday and ends by 3 p.m.

The District government has activated its hurricane plan, placing workers in the Department of Public Works and the Department of Transportation on notice for deployment on Sunday, said Pedro Ribeiro, a spokesman for Mayor Vincent C. Gray (D).

City officials were staying in close communication with the National Oceanic and Atmospheric Administration, Ribeiro said. “It’s not a matter of whether we’ll get hit; it’s a matter of how severe we’ll get hit.”

PEPCO made preparations for dealing with downed trees and power lines and other potential power outage hazards.

The District Department of Transportation has contractors and crews ready and available to remove trees from key major roadways, said spokeswoman Monica Hernandez.

Based on forecasts available on Friday afternoon, DDOT is expecting the bulk of relief efforts to kick in Monday at 6 a.m., she said. Crews might not be able to move some downed trees until there is daylight in order to see if a live wire is tangled up with the tree.

Hernandez warned people not to approach or touch downed trees, advising them to call 311.

Like Maryland, Virginia declared a state of emergency in order to launch preparations for the storm, which Virginia Gov. Robert F. McDonnell (R) said would likely start impacting parts of the commonwealth Saturday night. Forecasters said snow is possible in southern and western Virginia.

“Based on current forecasts, the eastern third of Virginia could experience tropical storm force winds for more than 48 hours, several inches of rain and coastal flooding,” McDonnell’s office said. “Even inland areas of Virginia could see strong winds and significant rainfall. There is a strong possibility of extensive power outages.”

The Virginia National Guard said it has been authorized to place up to 300 members on state active duty for possible recovery operations. It said it would begin staging about 50 Army Guard and Virginia Defense Force personnel in the Eastern Shore area and that all were scheduled to be in place by Saturday evening.

O’Malley, issuing Maryland’s third disaster declaration of the year, said: “As Hurricane Sandy makes its way north, I urge all Maryland residents to prepare for extreme weather. I urge all Marylanders to review their family emergency plans, make sure their emergency supplies like batteries and water are fully stocked and to stay informed.”

Washington area residents can find information on cancellations and delays at Schools and organizations can list their delays and cancellations by registering at

Metro currently has no plans to cancel weekend track work impacting four of the rail system’s five lines, but “if conditions really deteriorate on Sunday, we may change that,” Metro spokesman Dan Stessel said.

The transit agency started its storm preparations Thursday morning, placing sandbags around vent shafts and station entrances and checking pumping stations that will funnel water out of the stations. Stessel urged riders to sign up for Metro Alerts, which advise travelers of disruptions or detours via e-mail or text message.

The Maryland Transit Administration put its standard weather plans into effect on Friday as it prepared to mobilize its operations and maintenance, spokesman Joe Sviatko said.

“As long as it’s safe, the goal is to keep everything open that we can during our normal operational hours,” Sviatko said.

The agency has additional diesel trains available for MARC’s Penn Line, which typically uses electric trains, to try to ensure that MARC service is available on Monday. MTA officials will assess the storm’s path on Sunday afternoon to figure out exactly what service will be available on the system’s buses, trains and light rail system.

The Virginia Railway Express plans to have full service on Monday, barring any major problems stemming from the weather, said Mark Roeber, a VRE spokesman.

VRE would likely cancel Monday service if the Office of Personnel Management announces that the federal government is closed, he said. Flooding or debris on the tracks could also snarl or cancel some VRE service.

The rail company has already prepared the crews needed in the wake of the storm, Roeber said. “It really comes down to just a matter of watching the weather right now and trying to gauge what we’re doing,” he said.

D.C. Public Schools officials also urged parents to check the school system’s Web site over the weekend for updates on possible delays or closures.

“We are taking every step necessary to ensure that our buildings are protected throughout the storm and ready to open on time,” spokeswoman Melissa Salmanowitz said in a statement.

Montgomery County Public Schools spokesman Dana Tofig said maintenance crews will be on standby if the storm takes down trees or damages buildings. If the county cancels school on Monday, he said, the decision will be made no later than 5 a.m.

“A lot of it depends on which track the storm takes and when it hits,” Tofig said. “There really isn’t much we can do except wait to see what happens.”

The two most important things parents can do to prepare, Tofig said, is to make sure they know how they’ll receive emergency notifications from the school system and to make sure schools have the most up-to-date cell phone or home telephone information from parents in case school closes early.

Tofig said parents should visit the emergency information page on the school system’s Web site to make sure they are signed onto to text, e-mail, or Twitter alerts related to the storm. The school system also has a recorded phone line parents can call at 301-279-3673 for notifications.

Prince George’s County schools are closed on Monday for teacher professional development. But school officials are asking parents, students and employees to sign up for e-alert — a program that sends text messages about emergencies, school delays and closings — in case schools are affected by the storm later in the week. E-alert can be accessed through the school system’s Web site.

Briant Coleman, the Prince George’s schools spokesman, said maintenance crews were checking each of the facilities, ensuring that low-lying areas were clear.

“Hopefully we don’t get too much, but if we do, we’re ready, Coleman said.

In Loudoun County, work crews were busy inspecting the roofs of public school buildings across the county. School system spokesman Wayde Byard said personnel would ensure that all the roof drains were clear before the arrival of the storm.

“You don’t want standing water on a flat roof — that could be very bad,” Byard said, adding that no decision on school closures would be made until the storm’s potential impact was clearer.

“We’ll wait until we have a really good idea what’s going on,” he said. “It’s been more than a decade since we’ve canceled for a hurricane, but we’re watching it very closely.”

There are cameras set up at many of the schools in the county, and they will be monitored over the weekend for storm activity, said Byard.

Loudoun residents can sign up for county alerts through the Alert Loudoun system.

Arlington County schools currently have no emergency plans in place and have not yet made decisions on possible school closures for Monday.

“No plans are currently in place. We’re watching the storm right now, and any cancellation decisions will be made on Sunday,” said Frank Bellavia, a spokesman for Arlington Public Schools.

Custodians and construction crews working at Arlington Public Schools were tying down loose equipment before leaving for the day Friday, while school administrators were meeting with county officials in the afternoon to coordinate storm preparations.

Arlington County is taking all the necessary precautions for emergency support functions, and officials are currently trying to determine whether or not they will open their emergency operations center, said Bonnie Regan, acting deputy director at the Arlington County Office of Emergency Management.

There are no cancellations currently in place, said Regan. Residents looking to keep up with the latest details from the county throughout the storm can go to the Arlington alert Web site, she said. The public information office has been updating the county Web site with storm preparedness information, Regan said.

Alexandria city dispatched transportation crews to clear storm drains and catch basins by removing leaves and any other debris. Crews have been out since Thursday and will continue to work through the weekend, officials said.

“Alexandria is experienced in dealing with storms, and we are fully prepared to handle this one,” said Tony Castrilli, a spokesman for the city.

Alexandria was supposed to begin its six-week leaf collection on Monday. But now the city is asking residents not to sweep their leaves into the streets as they normally would, but instead to bag them so that the city can collect them when the storm moves on.

Residents are also being asked to clean their gutters and storm drains.

The annual Del Ray Halloween Parade scheduled for Sunday on Mount Vernon Avenue, which attracts nearly 1,000 families each year, has been canceled. No other cancellations were immediately reported.

Montgomery County workers are inspecting Rock Creek, Clarksburg and other local areas where flooding could occur, said Chris Voss, the county’s homeland security director. The county has dozens of generators ready to keep traffic lights working, he added.

If the path of the storm continues, the county is expected to open three shelters and have two emergency strike teams ready with firefighters, police officers and Pepco workers, Voss said.

Voss said the county’s five early voting sites have been added to a priority list for restoring storm outages. He added that Pepco would not start work on these sites until health-care facilities, such as hospitals and senior centers, are up and running.

In Prince William County, school officials discussed contingencies and double-checked their community-alert plans, but they said it was too early to make any decisions.

Officials are looking ahead at any planned school activities or trips scheduled for the coming days in case they need to be canceled, said Phil Kavits, a spokesman for Prince Williams County schools.

The school system is using the storm as an opportunity to remind students and parents about the importance of signing up for the county’s messaging system, he said.

Prince William’s police department went on standby, readying equipment and monitoring regional calls with updates on the impending storm.

Barry S. Barnard, the police department’s acting chief, said officers were going through supply closets to make sure they were adequately stocked with flares, lighting devices, signs and traffic-control devices, among other items needed in emergency situations.

An emergency staffing plan is in place, Barnard said, adding that he can summon officers to arrive earlier than usual or deploy more if extreme weather causes problems. Officers who serve in different units can be reassigned, he said, and many have been alerted that they may be needed during the storm.

“We try and stay as informed as we can and do what we have to do to be prepared,” Barnard said.

In the District, Capital Bikeshare could remain open during the storm, depending on its severity, according to DDOT spokesman John Lisle.

“Our standing policy is to keep the system open as long as we can, and if we have to shut it down, to shut it down for as short a period as possible,” Lisle said.

If the system is shut down, Bikeshare users could still return bikes to available docks, but they wouldn’t be able to check a bike out, Lisle said.

Gloria Garrett, who manages the Palisades Farmers Market every Sunday in Northwest Washington, said she isn’t too worried about the storm.

“I just e-mailed everyone to be sure they have their weights” for their tents, Garrett said.

The market runs from 9 a.m. until 1 p.m., so Garrett figures they may just encounter some rain and wind before the severe weather hits later. And the storm could wind up being good for business, because people could decide to stock up before the storm, she said.

“I think there might be up to a 20 percent uptick in sales,” Garrett said.

Candace Wheeler, Susan Svrluga, Jeremy Borden, Mark Berman, Ovetta Wiggins, Aaron Davis, Lynh Bui, Caitlin Gibson and Nikita Stewart contributed to this report.