A dark-money group created by allies of President Joe Biden released an ad last month thanking Arizona senator Kyrsten Sinema for pushing to lower prescription drug prices — just before she told the White House she will oppose Democrats’ drug pricing plan.

While Sinema previously campaigned on a pledge to reduce drug costs, the freshman senator and one-time-progressive-turned-corporatist has emerged as the chief roadblock to Democrats’ plan to allow Medicare to use its bulk purchasing power to negotiate significantly lower prescription drug prices. Because Democrats hold a narrow 50-50 majority in the Senate, they cannot afford to lose any votes on the measure.

In early August, Building Back Together, a nonprofit created to support Biden’s agenda, announced it was launching a $10 million ad campaign to build support for Democrats’ infrastructure plans while lawmakers were out of town on a long summer recess period.