The Hill News

Sen. Pat Toomey (R-Pa.) says Rick Santorum's past endorsement of then-Sen. Arlen Specter (R-Pa.) is water under the bridge.

"@RickSantorum for Specter: wrong but ancient history. More importantly: @MittRomney’s strong pro-growth tax reform plan, very much like mine," Toomey tweeted Thursday.

During Wednesday's Republican presidential debate, Mitt Romney criticized Santorum for endorsing Specter over Toomey in a 2004 GOP primary. Specter later defected to the Democratic Party and helped pass the Obama administration's healthcare reform law, which all the Republican candidates have vowed to roll back.

On Wednesday, Romney unveiled new details of his tax reform plan, which proposes cutting individual rates across the board by 20 percent.

Toomey has not yet endorsed a Republican candidate for president.