WASHINGTON - The National Rifle Association is actively supporting a Republican congressional candidate in a Chester County-based district, adding a powerful but often-polarizing force to a contest where both candidates have tried to portray themselves as commonsense centrists.

The gun-owners' group has donated nearly $10,000 to Ryan Costello's campaign and has deployed field staff in the district working to rally voters behind the first-time House candidate.

Costello is the only congressional candidate in the Philadelphia region receiving the NRA's on-the-ground support, according to filings with the Federal Election Commission.

The NRA's effort in the moderate district stands in contrast with the action in a nearby Bucks County-based district, where on Thursday a national gun-control group, Americans for Responsible Solutions, is launching a television ad praising U.S. Rep. Mike Fitzpatrick (R., Pa.) for supporting stronger federal background checks on gun purchases.

Fitzpatrick is the first Republican in the country whom the group is supporting on TV in an attempt to counter the NRA's influence in elections.

The NRA's work for Costello highlights an issue that rose to the national forefront last year after the Newtown, Conn., school shootings prompted Sen. Pat Toomey (R., Pa.) and others to sponsor a plan to expand background checks for gun purchases.

That issue has not featured so far in Costello's contest against Democrat Manan Trivedi as they run to replace retiring U.S. Rep. Jim Gerlach (R., Pa.).

In the sprawling Sixth District, the NRA could bring out passionate gun owners in its more rural stretches, but may also give pause to suburban moderates.

Costello is "a candidate we're backing based on his solid support for our issues," Catherine Mortensen, a spokeswoman for the NRA's lobbying arm, said in an interview. "He filled out the NRA questionnaire that we give to all candidates, and he's a very strong candidate on the Second Amendment."

The group donated $4,950 to Costello's primary campaign - the most it gave to any Pennsylvania congressional race as of June 30, the latest reporting date, according to the nonpartisan Center for Responsive Politics. The NRA added another $4,950 for the fall election, Mortensen said.

In addition, field staff in the district have worked to build Costello support, she said. The NRA has spent about $5,250 since June on so-called independent expenditures to aid Costello. Such spending can support a candidate, but cannot be coordinated with his or her campaign.

A Costello spokesman said the candidate had no knowledge of the NRA's activity on his behalf. The campaign declined to make him available for an interview, and pointed out that the NRA donations were a small portion of the more than $1 million Costello has raised.

"Ryan is a proud supporter of the Second Amendment, and his record on public safety is second to none," Costello spokesman Pete Peterson wrote in an e-mail. He called Trivedi "an enemy of lawful gun owners" and said Costello, "like other members from Southeastern Pennsylvania, supports commonsense federal gun legislation that mirrors the laws currently in place in Pennsylvania."

Pennsylvania's existing laws, though, stop short of the background-check proposal introduced by Toomey and endorsed by Fitzpatrick and U.S. Rep. Patrick Meehan (R., Pa.).

Toomey's failed bill, a compromise crafted with Sen. Joe Manchin (D., W.Va.) would have required background checks on most long-gun sales - which are not covered by Pennsylvania law. The state requires background checks on handgun sales.

Pressed on this point, Peterson said that if the Toomey-Manchin bill came to a vote, Costello "would likely support it."

Democrat Trivedi, in an interview, said he respected gun owners' rights, but "as somebody who served in the military and Iraq and as a doctor who has worked in emergency rooms ... I know how critically important responsible gun safety is."

He said he would support bills to expand background checks and ban assault weapons.

The NRA brings benefits and risks for Costello, said Chris Borick, a political scientist at Muhlenberg College.

"For a lot of core Republican voters, gun rights remain one of the most important issues on the table," he said, and the NRA is renowned for rallying its supporters to the polls.

But in wealthier suburbs where many of the district's voters live, "an NRA message might be more divisive."

The group could bring out gun owners for Costello without the candidate's having to actively campaign on an issue that might turn off moderates, Borick said, helping in "a targeted, limited way."

The NRA fiercely opposed Toomey's plan, though gun-control groups say their polls show background checks are overwhelmingly popular in suburban districts such as those around Philadelphia.

In Bucks County, Fitzpatrick was one of the few House Republicans to back Toomey's plan. He cosponsored companion legislation, though it never got a hearing.

Fitzpatrick's stand won him support from Americans for Responsible Solutions, the group founded by former U.S. Rep. Gabby Giffords (D., Ariz) after she was shot in a 2011 rampage in Tucson.

Fitzpatrick is one of just two Republicans the gun-control organization is backing.

"Fitzpatrick isn't afraid to lead," says the group's 30-second television ad. "He's working with Democrats and Republicans to keep us safe from gun violence."

His Democratic challenger, Kevin Strouse, strongly supports "comprehensive background checks," spokesman Brendan McPhillips wrote in an e-mail.

@JonathanTamari