They clashed on Capitol Hill over bankruptcy reform in 2005.
And met again at a swearing-in ceremony for new Senators in 2013.
But a years-long rivalry between Democratic presidential frontrunner Joe Biden and the increasingly popular Elizabeth Warren could come to the fore on Thursday, when the two meet for the first time on the debate stage in Houston, Texas.
(SOUND BITE) (ENGLISH) MASSACHUSETTS SENATOR ELIZABETH WARREN, SAYING (SEPTEMBER 10, 2019): "I don't want a government that works for giant corporations.
I want one that works for our families." In the early 2000s, Warren accused Biden of supporting big corporations, saying a bill he backed to overhaul the nation's bankruptcy laws gave more power to credit card companies and hurt financially distressed women and families.
(SOUND BITE) (ENGLISH) FORMER VICE PRESIDENT JOE BIDEN, SAYING( MAY 18, 2019): "I know how to make government work." Seen as a centrist candidate, the former vice president has touted his long career in government, casting himself as a champion of compromise.
(SOUND BITE) (ENGLISH) FORMER VICE PRESIDENT JOE BIDEN, SAYING (JUNE 4, 2019): "If you notice, every single time we had a problem in the administration, who got sent up to Capitol Hill to settle it?
Not a joke, me." But on Thursday, the more progressive Warren is likely to make her case for changing a political system in Washington that, she argues, doesn't work well enough for most Americans.
(SOUND BITE) (ENGLISH) MASSACHUSETTS SENATOR ELIZABETH WARREN, SAYING (SEPTEMBER 7, 2019): "We start by calling out honestly what is broken - the corruption in the system - and, then, we show we've got plans to fix it." The two will share the stage with eight other presidential contenders, including senators Bernie Sanders, Kamala Harris, Cory Booker and Amy Klobuchar, as well as South Bend, Indiana, Mayor Pete Buttigieg, former congressman Beto O'Rourke, entrepreneur Andrew Yang, and former Obama cabinet member Julian Castro.
While Biden is still ahead of the pack, a Reuters/Ipsos poll released Wednesday shows support for Warren rose more than that of any other Democratic presidential hopeful over the past month, solidifying her status as a top contender to challenge Trump in 2020.
And the president himself has weighed in on Warren gaining on Biden in the polls.
(SOUND BITE) (ENGLISH) U.S. PRESIDENT DONALD TRUMP, SAYING (AUGUST 13, 2019): "She's staging a comeback on Sleepy Joe.
I don't know who's going to win but we'll have to hit Pocahontas very hard again if she does win.
But she's staging a little bit of a comeback.
What a group: Pocahontas and Sleepy Joe."