Former Nebraska Republican Sen. Chuck Hagel will formally endorse Pennsylvania Democrat Rep. Joe Sestak for Senate in Pittsburgh and Philadelphia on Tuesday.

Sestak, a Navy veteran who rose to the rank of admiral and served in Iraq, is facing former GOP Rep. Pat Toomey in November.

Hagel, a Vietnam veteran, was known as a maverick during his two Senate terms. He was a senior member of the Foreign Relations Committee when he retired in 2008 and an outspoken voice for leaving Iraq. He declined to publicly support Sen. John McCain (R-Ariz.) or then-Sen. Barack Obama (D-Ill.) in the 2008 presidential election, but his wife Lilibet endorsed Obama in October.

Sestak defeated longtime Sen. Arlen Specter, who left the GOP in 2009, in the Democratic primary in May. Sestak became a Member of Congress in 2006, when he beat Republican Rep. Curt Weldon in a district just west of Philadelphia.