BRIDGEVILLE, Pa. — Conor Lamb, the Democratic nominee in a March 13 special congressional election in southwest Pennsylvania, has deviated from his party’s positions on guns — and a Republican super PAC wants Democratic voters to know about it.

An ad arriving by mail offers a backhanded “thank you” to Lamb from Congressional Leadership Fund, a super PAC aligned with House Speaker Paul D. Ryan (R-Wis.) that has spent millions of dollars on the race.

The same group then turned around and sent GOP-leaning voters a very different spin on Lamb — calling him an opponent of gun rights.

“Typical politicians like Paul Ryan and Rick Saccone lie to us every day, but they’re not fooling us,” said Lamb campaign manager Abby Murphy. “Conor Lamb supports strengthening universal background checks. Rick Saccone wants to eliminate background checks. It’s that simple.”

In his first television ad, Lamb said he learned to use firearms in the Marines, and “still loved to shoot.” After the mass shooting in Parkland, Fla., Lamb said at a debate, and in a Q&A with reporters, that he favored stricter background checks but wasn’t sold on other new gun laws.

“The emotions that a lot of us are feeling right now are very raw because we know that there’s not one thing we can do with the stroke of a pen or one thing you can ban,” he said.

One week later, Democrats in Allegheny County began receiving an unusual piece of mail. Congressional Leadership Fund, or CLF, had created an advertisement thanking Lamb “for supporting our Second Amendment rights.”

It’s not clear who else is getting the ad, but registered Democrats in Allegheny County are, generally, more liberal than their counterparts across the district.

“Our job is to make sure voters in Pennsylvania’s 18th District know where Conor Lamb stands on important issues of the day,” said Courtney Alexander, a CLF spokeswoman. “This mailer was sent to voters who might find his views on the Second Amendment of note.”

The 18th District, which will elect its last congressman this month before a new court-drawn map takes effect, covers some suburban parts of populous and left-leaning Allegheny County, then cuts across the much more conservative Greene, Washington and Westmoreland counties. Registered Democrats outnumber registered Republicans in the district, but voters there gave Donald Trump a 19-point landslide win in 2016. Many of the Democrats who still hold local office in the district oppose abortion and gun control.

Conservative voters have gotten a very different message from CLF.

In a door-hanger ad that the PAC has given Republican-leaning voters, images of Hillary Clinton and Rep. Nancy Pelosi (D-Calif.) accompany a warning that Lamb “wants to impose restrictions on gun ownership and the Second Amendment.” Rick Saccone, the GOP nominee, has been endorsed by the National Rifle Association, in part owing to his efforts to unravel some Pennsylvania gun laws.

Ads designed to play to the other party’s voters are hardly new.

In 2016, the Democratic Congressional Campaign Committee intervened in a Nebraska primary to boost a Republican candidate opposed by the party’s establishment, hoping to weaken eventual nominee Don Bacon. (Bacon won the nomination and then, narrowly, the election.) In this cycle, CLF President Corry Bliss has said he wants to intervene to help more left-wing candidates in Democratic primaries, on the theory that they’d be weaker general election nominees in swing districts.