WASHINGTON -- Sen. Bob Casey endorsed fellow Democrat Katie McGinty in Pennsylvania's U.S. Senate primary Friday, calling her their party's best chance to unseat Republican Sen. Pat Toomey this fall.

"The challenges we face this year and the likelihood of getting a Democratic victory in November is too significant to stay on the sidelines," Casey said in a morning call with reporters. "I think she will be the strongest nominee in the fall election against Sen. Toomey, and that's part of the determination that I have to make as a Democratic leader in the state but also as a voter."

Casey chose McGinty over Joe Sestak, a former admiral and Congressman from Delaware County who has had an edge in public polling, but whose campaign style worries Democratic leaders in Washington and Pennsylvania. The party establishment has long sought an alternative for a critical race that could decide control of the chamber.

McGinty, Sestak, Braddock Mayor John Fetterman and Joseph Vodvarka are competing in the Democratic primary for the chance to challenge Toomey.

McGinty, Gov. Wolf's former chief-of-staff, has won widespread support from the Democratic establishment – including from Wolf, the Senate's top Democrat, Harry Reid, and major labor unions – but until Friday Casey stayed quiet.

Even before it was official, Sestak cast the endorsement as another sign of his independence.

"I have had no politician's endorsement in this campaign," Sestak said in a news release. "With Bob Casey's endorsement of my primary opponent today, it completes an all-inclusive rejection by Washington DC's and Pennsylvania's Democratic politicians of what I believe in, and stand for."

A similar situation played out in 2010, when the Democratic establishment – including Casey – endorsed the late-Sen. Arlen Specter in a Democratic primary. Sestak won that race, but lost a close general election to Toomey.

Casey, who met with Sestak when the former congressman launched his campaign early last year, said he spoke now because voters are just tuning into the race, which is now just weeks away from the April 26 primary.

Casey said he knows McGinty the best of all the Democratic candidates – she is a former environmental aide in the Clinton White House and ex-Gov. Rendell's administration -- and admires her record and the issues she is campaigning on, such as raising wages, ensuring equal pay for women and trying to make college affordable.

"I think she will win the primary, I think she'll be elected in November," Casey said.

Despite McGinty's wave of endorsements, though, she has not caught up to Sestak in public surveys.

Many voters she said, are still undecided and most polls show the primary remains close. McGinty argued that Sestak's early advantage is due to his name recognition as a 2010 Senate candidate who has barnstormed the state for years, and claimed momentum is on her side.

"Today we just get a big battery pack strapped to a campaign that's already been charging ahead at a pretty good clip," McGinty said of Casey's support. Both campaigns have taken to the airwaves in recent weeks.

Sestak has a following among grassroots Democrats, but party leaders chafe at his independent streak and tendency to keep his own counsel, running campaigns that cut against the advice of professional political operatives.

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