Pennsylvania Sen. Bob Casey, a Democrat, endorsed Hillary Clinton for president Friday in an email to supporters, calling her an advocate for working Americans.

"Hillary has dedicated her life to public service and the fight for everyday Americans," Casey said. "Having served with her in the Senate, I know she has always been a strong advocate for the middle class and I'm confident she will work tirelessly to ensure that Pennsylvania families have the chance to get ahead and stay ahead."

Casey sided with then-Sen. Barack Obama in the grinding 2008 primary with Clinton, and campaigned with Obama during the six weeks leading up to the climactic Pennsylvania primary. Clinton carried the state, but lost the nomination.

It's not as if Casey had any viable alternative choices at this point. Clinton, former first lady, U.S. senator from New York and secretary of state is the only announced candidate for the Democratic nomination. Some on the left have pushed for another choice, citing Clinton's ties to Wall Street and generally moderate views.

Former Maryland Gov. Martin O'Malley is considering a challenge to Clinton as are former Virginia Sen. Jim Webb and Vermont Sen. Bernie Sanders, an independent who caucuses with the Democrats. Clinton's lead over any potential primary opponent tested is astronomical in recent polls.