If he wants to remain in the Senate race, Rep. Joe Sestak (D-Pa.) should resign "so he can cease being a burden to the taxpayers," his primary opponent Sen. Arlen Specter (D-Pa.) said on Thursday.

The alternative, according to Specter, would be to "go back to work in the House full time" and butt out of the Senate campaign.

Specter's comments represent his harshest attack on Sestak yet. In particular, the Republican-turned-Democrat has consistently criticized Sestak for skipping over 120 votes since the beginning of this year.

The veteran senator claims that many of the absences have been campaign-related.

"You are asking the voters for a promotion but are failing to perform the most basic duty of any Congressman, which is to vote on the floor of the House of Representatives," Specter wrote in a letter to Sestak dated today.

An interesting side note: In a tweet today, Specter accuses Sestak of missing 125 votes, while the letter claims he has skipped 122.

The two campaigns have often engaged each other using fiery rhetoric. Sestak has attacked Specter's votes during his time as a Republican senator.

The winner of the primary is likely to face Republican front-runner and former Rep. Pat Toomey (R-Pa.) A recent Quinnipiac poll shows Specter leading Sestak by a wide margin. But Toomey is running strong in a hypothetical match-up with either opponent.