2016 hopefuls celebrate Harkin's career
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Three 2016 presidential hopefuls late Friday celebrated Sen. Tom HarkinTom HarkinGrassley challenger no stranger to defying odds Clinton ally stands between Sanders and chairmanship dream Do candidates care about our health or just how much it costs? MORE (D-Iowa), who will retire from Congress at the end of the year after serving for 40 years.

Maryland Gov. Martin O’Malley (D) delivered his speech praising Harkin, 74, in person at the Iowa Democratic Party’s Hall of Fame dinner. Hillary ClintonHillary Rodham ClintonIll. gov candidate runs as fresh face, despite ties to political machine Huma Abedin 'working hard' on marriage with Anthony Weiner: report Mark Cuban: My political future 'depends on how things play out' MORE and Vice President Joe BidenJoe BidenBiden: I regret not being president Biden: 'McCain is right: Need select committee' for Russia With no emerging leaders, no clear message, Democrats flounder MORE also paid tribute. Iowa is the first state to kick off the presidential primary season. 

Harkin's success "can be measured by the millions of Americans whose lives he touched … those who are in the dawn of life, children; those who are in the twilight of life, the aged; and those in the shadows of life, the sick and the needy and the handicapped,” O’Malley said, according to The Des Moines Register.

At the dinner, Harkin called O’Malley “a great progressive Democrat and someone we can look up to as we move forward in America." O’Malley has voiced strong interest in a White House bid in 2016.

Clinton, considered the Democratic frontrunner if she decides to run, paid tribute to Harkin in a pre-recorded video in which she asked for grandparent-related advice. Clinton's daughter Chelsea is due to have her first child this fall. 

“No one knows the terrain better,” Clinton said than Harkin. “So Tom, I need to ask: What's the secret to being such a terrific grandparent?" 

In a pre-recorded video message by Biden, he called his old Senate colleague “the conscience of the Democratic caucus” and said he will cherish the last six months they’ll have working together in Washington. Biden could also launch a presidential bid. 

"Tom, you've changed America," President Obama chimed in, in another video. "You and your family have a special place in my heart. And of course, so does Iowa. It was at Tom's legendary steak fry where our march for change began."

Harkin served in the House for 10 years and has served 30 years in the Senate.