2016 hopefuls celebrate Harkin's career
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Three 2016 presidential hopefuls late Friday celebrated Sen. Tom HarkinTom HarkinThe Hill's 12:30 Report Distance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds 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 Diane Rodham ClintonO’Malley tells Dems not to fear Trump FBI informant gathered years of evidence on Russian push for US nuclear fuel deals, including Uranium One, memos show Pelosi blasts California Republicans for supporting tax bill MORE and Vice President Joe BidenJoseph (Joe) Robinette BidenObama tweets birthday message to Biden: 'The best vice president anybody could have' The Hill's 12:30 Report Tech beefs up lobbying amid Russia scrutiny 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.