Outgoing Sen. Arlen Specter (D-Pa.) on Monday will campaign for the man who ended his three-decade Senate career, Rep. Joe Sestak (D-Pa.)

Sestak defeated the Republican-turned-Democrat in a hotly contested primary race in May, but the two will put their differences aside at a event in Philadelphia on Monday, according to the Philadelphia Jewish Voice. The event will be the first time Specter appears on Sestak's behalf.

Specter is scheduled to appear alongside Sens. Chuck SchumerCharles (Chuck) Ellis SchumerSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill This week: Senate wrapping up defense bill after amendment fight Cuomo warns Dems against cutting DACA deal with Trump MORE (D-N.Y.) and his Pennsylvania counterpart, Sen. Bob CaseyRobert (Bob) Patrick CaseySenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill GOP eying 'blue slip' break to help Trump fill the courts Dems offer alternative to Trump administration's child care proposal MORE (D), in an effort to boost the second-term congressman, who trails his Republican opponent former Rep. Pat Toomey in the polls.

There is no love lost between Specter and Toomey. Toomey challenged Specter in the 2004 GOP Senate primary but was defeated by the incumbent. 

The centrist Specter switched parties in April 2009, saying the Republican Party had moved too far to the right, but he also admitted the move would allow him to avoid a difficult primary match-up with Toomey in 2010. 

Specter received President Obama's endorsement during the Democratic primary, but was defeated by Sestak in an anti-incumbent year. Sestak argued he is an independent voice and a more genuine Democratic candidate than Specter, who was a Republican for 44 years.

Specter said during the primary that he would appear for Sestak if he lost, but last month ducked a question about his chances of winning, saying "I'm late for the squash court."

The five-term Pennsylvania senator is one of three incumbent senators to lose a primary election this cycle. One of them, Sen. Lisa MurkowskiLisa Ann MurkowskiSenate Dems hold floor talk-a-thon against latest ObamaCare repeal bill Collins skeptical of new ObamaCare repeal effort How Senate relationships could decide ObamaCare repeal MORE (R-Alaska), is running for reelection as a write-in candidate.