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. Charles SchumerCharles (Chuck) Ellis SchumerOvernight Finance: White House planning new tax cut proposal this summer | Schumer wants Congress to block reported ZTE deal | Tech scrambles to comply with new data rules OPEC and Russia may raise oil output under pressure from Trump The Hill's 12:30 Report — Sponsored by Delta Air Lines — GOP centrists in striking distance of immigration vote MORE (D-N.Y.) and his Pennsylvania counterpart, Sen. Bob CaseyRobert (Bob) Patrick CaseySenate Dems to Mnuchin: Don't index capital gains to inflation Senators offer tax bill aimed at helping first responders McConnell: Midterms will be 'very challenging' for GOP 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 MurkowskiGOP senators introduce Trump's plan to claw back billion in spending Overnight Health Care — Sponsored by PCMA — VA reform bill heads to Trump's desk Senators introduce bill to measure progress in opioid fight MORE (R-Alaska), is running for reelection as a write-in candidate.