House Republican leaders’ main political action committee rushed in Wednesday with ads to bolster another Virginia Republican congressman, in the latest sign that the state is ground zero in the battle for the House majority.

The late-October move in Virginia’s 5th District, which President Trump carried by double digits in 2016, means Republicans are playing defense in areas that used to be reliable strongholds.

A public poll shows Democratic candidate Leslie Cockburn up by 1 percentage point over Republican nominee Denver Riggleman in a race that has Republicans scrambling to fend off disaster in the Old Dominion.

As many as four of the seven Republican-held seats in Virginia could be in play, making the state one of the biggest prizes on the board as Democrats try to net the 23 pickups needed to regain control of the House.

“This last-minute financial lifeboat from Paul Ryan’s super PAC will not be enough to save Denver and his lackluster, out-of-touch campaign,” said Jake Rubenstein, a spokesman for the Democratic Party of Virginia.

A Republican Party official characterized the PAC’s decision to jump into the 5th District, which runs from the North Carolina border up through Charlottesville into the exurbs of Washington, as a standard move in a competitive race.

The Congressional Leadership Fund, one of the top outside groups in terms of spending this cycle, also announced new ad campaigns this week in a number of other races, including conservative-leaning districts in states such as Kentucky and New York.

“We’re watching [Virginia’s 5th District] like we’re watching all of our other races,” the official said.

The New York Times/Siena College poll conducted from Oct. 16-22 showed Ms. Cockburn leading Mr. Riggleman 46 percent to 45 percent after prognosticators rated the race as leaning toward Mr. Riggleman.

But that’s better than two toss-up races in the districts of Reps. Dave Brat and Scott Taylor, and the district of Rep. Barbara Comstock, a Republican whom analysts expect to lose in her suburban Washington district.

The Republican leadership PAC’s entry is an indication that even if Mr. Riggleman ekes out a win, the race has become competitive to the point where Republicans can’t take it for granted, said Quentin Kidd, a political science professor at Christopher Newport University in Newport News.

“Part of this is bet-hedging,” Mr. Kidd said. “Republicans are probably basically saying, ‘If it’s going to get that close, given the uncertainty of polling sometimes, let’s go ahead and put some money in here to try to make sure that we’re comfortably safe in this seat.’”

He said Democrats are likelier to unseat Ms. Comstock, who is running against state Sen. Jennifer Wexton in the 10th Congressional District.

A recent NYT/Siena poll showed Ms. Wexton leading by 7 points, though the Comstock campaign has said she is running strong in her internal polls and has never underperformed in those. She won re-election in 2016 by 6 points, even as Hillary Clinton carried the district by double digits over Mr. Trump.

In Mr. Taylor’s race in the 2nd District, a CNU poll showed him leading Democrat Elaine Luria in a district that Mr. Trump carried by about 3 points in 2016.

Mr. Brat, who toppled former House Majority Leader Eric Cantor in a Republican primary in 2014, also has found himself with a fight on his hands in his race against Abigail Spanberger in the 7th District.

Mr. Brat said recently that Republicans are coming back home after the confirmation hearings of Supreme Court Justice Brett M. Kavanaugh, whose forceful denials of past sexual misconduct energized many conservatives looking for a reason to get to the polls next month.

“We do have people coming our way now. … The Republicans are coming home, independents are breaking our way after the Kavanaugh story, and so in the end I think Virginia people are going to do the right thing,” Mr. Brat told radio host John Fredericks.

Still, the Democratic candidates and their allies in the 2nd, 5th and 7th District races all outspent their Republican counterparts in broadcast TV ads last week, according to data released Wednesday by the Virginia Public Access Project.

For the first time since Labor Day, Ms. Comstock and her allies outspent the Democrats on broadcast TV ads last week, according to the project — an indication that Republicans might not be willing to give up on that race.

Democratic Sen. Tim Kaine is heavily favored to win a second term this year, and some Republicans have griped that Republican challenger Corey A. Stewart’s lackluster campaign could have a negative down-ballot effect for Republican incumbents in the House.

The bomb-throwing Mr. Stewart has bear-hugged Mr. Trump and has continued to make defending Confederate monuments in the state a key part of his campaign. That message dated back to his unsuccessful run for governor last year.

Republicans haven’t won a statewide race in Virginia since 2009 — though political scientist Stephen Farnsworth said Virginia should still be considered a purple state.

“But if Republicans plan to win statewide elections, they have to offer purpler candidates,” said Mr. Farnsworth, a professor at the University of Mary Washington. “Virginia has changed a lot since George Allen ran for governor, and the messages that worked then won’t work now for the Republicans.”

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