WASHINGTON — In an unusual twist, a major conservative group is taking aim at U.S. Rep. Lou Barletta, the Pennsylvania Republican who is the party's nominee in a key U.S. Senate race.

Americans for Prosperity, which supports limited government, is taking out ads targeting 17 House members in both parties who backed a $1.3 trillion spending bill Congress passed in March. The effort could ding Barletta, a GOP underdog battling U.S. Sen. Bob Casey (D., Pa.) — and counting on conservative advocacy groups to help counter a significant fund-raising gap. Instead, Barletta is taking friendly fire from an expected ally seeking to punish lawmakers who it argues have strayed from conservative fiscal principles.

The only other Pennsylvanian targeted is U.S. Rep. Matt Cartwright, the state's most vulnerable Democratic congressman. The ads are set to begin running over Memorial Day weekend

While Americans for Prosperity did not say how much it is spending in Pennsylvania, the attack on Barletta is notable because of the group's typical support for Republicans. The organization, backed by the billionaire industrialist Koch brothers, campaigned heavily in 2016 for Pennsylvania Republican Sen. Pat Toomey, who won a narrow reelection.

Barletta took the ads as a positive.

"I have Bob Casey attacking me from the left, and the Koch Brothers from the right. If the far left and far right are against me, I must be doing the right thing," he said in a statement from his campaign.

Americans for Prosperity plans to run online ads and send mailers criticizing Barletta, and to go after Cartwright with mailers and ads on radio and online. Cartwright is running in a Northeast Pennsylvania district that President Trump won by about 10 percentage points.

Casey is widely favored, although Republicans hope to unseat him by duplicating Trump's 2016 Pennsylvania victory. Several other Pennsylvania Republicans also voted for the spending plan, but have not been targeted by AFP.

The mailer says that Barletta "voted to increase waste that helped push spending to more than $4 trillion this year alone."

A spokesman for the group said it chose to target races where it believed it could "elevate" the spending issue.

The White House supported the spending plan, which helped avert potential government shutdowns, only for Trump to threaten to veto it. He still signed it into law.

At the time, Barletta said the bill wasn't perfect but pointed to its increase in funding for the military and $4 billion to fight the opioid crisis.