Indeed, a recent Rasmussen poll indicates Castle — a nine-term congressman with a long independent streak — is a 12-point favorite over the Democratic candidate, Chris Coons. O'Donnell, meanwhile, is running 10 points behind Coons, Rasmussen found.
Delaware's Republican Senate primary is slated for Sept. 14.
At issue is Tuesday's House vote on a $26.1 billion state-aid package, which included $16.1 billion in emergency Medicaid funds to help states balance their budgets in a tough fiscal year. Though the bill was fully offset, most Republicans opposed the measure, arguing that the government simply can't afford more stimulus spending in the face of record deficits.
Two Republicans broke with party leaders to vote in favor of the bill: Castle and Rep. Joseph Cao (La.). Three Democrats opposed the measure, which President Obama signed into law Tuesday evening.
"It's time to stop the madness," O'Donnell wrote. "The best thing the government can do to begin REAL economic recovery is to get out of the way of the small business owner and the entrepreneur."
Castle's office did not respond immediately to a request for comment Thursday.
If the issue has been partisan in Washington, in the states it's been anything but.
Earlier in the year, a long, bipartisan list of state governors urged Congress to approve an even more generous increase in Medicaid's Federal Medical Assistance Percentages (FMAP).
“The length and depth of the recession means states and territories will continue to face significant budget shortfalls long after the enhanced FMAP provisions expire at the end of this calendar year,” wrote the governors, representing 42 states and five U.S. territories.
Delaware Gov. Jack Markell (D) was among the signatories.