Republican Ed Gillespie, left, and Democrat Ralph Northam are neck and neck in the race for Virginia governor. (Bill O'Leary/The Washington Post)

The Las Vegas mass shooting, the deadliest in modern U.S. history, pushed gun control to the front of the neck-and-neck race for Virginia governor on Monday.

At a previously scheduled forum in Vienna that was held hours after a gunman killed at least 58 people and injured hundreds, Democrat Ralph Northam and Republican Ed Gillespie offered condolences.

But the partisan divide over guns in Virginia, a Southern state with a strong gun tradition that was shaken by the 2007 Virginia Tech massacre, was immediately evident.

Northam, a former Army doctor who has an F rating from the National Rifle Association, decried what he called "a proliferation of guns" in society and urged gun-control measures.

Gillespie, who has an A rating and an endorsement from the NRA, asked for a moment of silence, later telling reporters that it was too soon to discuss policy.

Former congresswoman Gabrielle Giffords (D-Ariz.), who became one of the nation's most prominent gun-control advocates after she survived a 2011 shooting in her district, canceled plans to campaign for Northam on Monday in Virginia.

Northam said the cancellation was out of "sensitivity" to grieving families, although Giffords and her husband, retired astronaut Mark Kelly, responded to the Las Vegas violence at an afternoon news conference outside the U.S. Capitol.

"Politics matters," Kelly said. "Who we elect matters. Don't let anyone tell you not to talk about politics when we talk about guns."

Americans for Responsible Solutions, Giffords's gun-control group, has pledged to spend $150,000 on pro-Northam mailings.

Northam is also backed by Everytown for Gun Safety Action Fund, a group bankrolled by former New York City mayor Michael Bloomberg (I) that is spending at least $700,000 on his campaign.

"In light of the tragedy in Las Vegas, today is a day for resolve," said Kate Folmar, an Everytown spokeswoman. "In this governor's race, Virginians have a clear choice between a candidate who believes we must do much to prevent such tragedies, Ralph Northam, and a candidate who does not, Ed Gillespie."

View Graphic Virginia general election guide

Meanwhile, the NRA, which had planned to run political advertising in Virginia starting Tuesday through Election Day on Nov. 7, delayed those commercials for one week, according to a media buying firm and a Richmond television station.

The gun rights group plans to spend more than $750,000 on commercials in the Richmond and Roanoke markets, according to filings reviewed by the nonpartisan Virginia Public Access Project.

It's unclear what they are about; a spokeswoman for the NRA did not respond to requests for comment.

The delay in the NRA ads is not connected to the shooting in Las Vegas, said a person familiar with the ad buy who was not authorized to talk on the record and spoke on the condition of anonymity.

This is the second time an episode of gun violence has had ripple effects on the Virginia governor's race, the nation's marquee statewide election this year.

The day after the June 13 primary, a gunman opened fire at a congressional softball practice in Alexandria, wounding House Majority Whip Steve Scalise (R-La.) and four others. Democrats canceled a planned unity rally, while Republicans scrapped a victory tour in favor of a prayer in Fairfax.

[Rep. Steve Scalise returns to Congress 15 weeks after he was shot]

Northam said Monday that the latest tragedy required more than "thoughts and prayers."

"There is a tremendous proliferation of guns throughout this country, and there are tragedies that occur every day because of gun violence that we don't even hear about," Northam said in his speech at the forum in Vienna. "We as a society need to stand up and say it's time to take action, and stop talking."

He later told reporters that such actions in Virginia include requiring universal background checks for firearm purchases and reinstating a Virginia law that restricts purchases to one handgun per month. That law expired in 2012.

Northam was scheduled Monday night to appear at a candlelight vigil for Las Vegas victims outside Alexandria's city hall with Lori Haas, the mother of a Virginia Tech survivor and leader of the Virginia branch of the Coalition to Stop Gun Violence.

After his forum appearance, Gillespie declined to answer questions from reporters about whether Virginia should change gun laws in light of the latest mass shooting.

"The appropriate comments and appropriate response right now is to take a moment and pray. And that's what I'm doing," said Gillespie, who appeared emotional. "There will be ample time for conversations and policy discussions and politics over the course of this campaign."

Gillespie is opposed to further restrictions on guns and promised to reverse an executive order signed by Gov. Terry McAuliffe (D) that bans guns in state government buildings.

Guns have long been a major factor in political races in Virginia, home to the NRA headquarters. Before a gunman killed 49 people in Orlando last year, Virginia had the dubious distinction of being the site of the deadliest mass shooting in modern history when 32 people died in the Virginia Tech massacre.

After long avoiding gun control, Democrats running for statewide office in Virginia in the aftermath of Virginia Tech and other mass shootings began calling for tougher laws — and found they could still win.

McAuliffe, who is barred from seeking consecutive terms under the state constitution, embraced his F rating from the NRA when he ran in 2013. Two previous Democratic governors, Sens. Tim Kaine and Mark R. Warner, were rated F and C, respectively, by the NRA during their most recent election campaigns.

Asked about the NRA's upcoming ad buy, Kaine called the Fairfax-based NRA a "paper tiger."

"The problem with the NRA, bluntly, is that they've morphed from an organization that reflected the views of their members to an organization that just does the bidding of gun manufacturers," said Kaine, who joined Warner on Monday at a previously scheduled Northern Virginia Chamber of Commerce event in Falls Church. "And gun manufacturers have a basic rule: whenever, wherever, however, to whomever in whatever quantity. That's what they want."

Kaine said he is opposed to a bill pending in Congress that would make it easier to put silencers on weapons. In the shooting in Las Vegas, the sound of the gunfire apparently helped people know where to go to avoid more fatalities, he said.

[How Tim Kaine’s positions on guns evolved after Virginia Tech ]

Mike DeBonis and Jenna Portnoy contributed to this report.