From Washington, Schumer oversaw the Minnesota Democrat's first campaign in 2008. When Franken was finally declared the winner in the summer of 2009 after a long recount, he gave the Democrats a brief filibuster-proof majority of 60 seats - a precious gift that allowed them to pass the Affordable Care Act on a party-line vote that December.
The experience forged a strong friendship between the two that has lasted ever since. But this week's allegations that Franken engaged in inappropriate advances before joining the Senate puts their bond to a new test. Coming amid a national focus on other powerful men accused of sexual improprieties, Franken quickly found himself under fire from Republicans and many Democrats.
Younger and newer Democrats, in particular, showed no deference. "Sexual harassment, misconduct, should not be allowed by anyone, and it should not occur anywhere against anyone," said Sen. Kamala Harris, D-Calif., a rising star in her first year in office.
Sen. Kirsten Gillibrand, D-N.Y., who has helped lead the push for stricter rules on harassment in Congress, immediately called for an ethics investigation. Gillibrand, 50, and Harris, 53, both considered future presidential contenders, made their remarks less than an hour after the news broke about the allegation that Franken had groped a fellow entertainer on a United Service Organizations trip to Iraq in 2006. The senators did not wait to consult with Schumer before denouncing Franken.
The allegations against Franken come at a time of broad awakening on the issue of sexual misconduct, with accusations surfacing in recent weeks against film producer Harvey Weinstein, Republican Senate candidate Roy Moore of Alabama and others. As the news sank in about Franken, even some veterans began adding his name to that growing list.
"I think that people feel they can get away with a lot of things," said Sen. Dianne Feinstein, D-Calif. "And nobody's going to pay any attention."
Feinstein was among four women elected to the Senate in 1992 - a momentous enough occasion that the cycle was dubbed the "Year of the Woman." Here's another important detail about that wave: It came after the then-all-male Judiciary Committee effectively ignored allegations that Supreme Court Justice Clarence Thomas had sexually harassed Anita Hill.
Even before this recent wave of allegations, a huge number of female candidates had signed up to run for Congress and other public offices, at first in reaction to Trump's presidency. Perhaps more will come forward in response to these stories. "In a way, it's a continuation of the Year of the Woman," Feinstein said.
Some things have changed since 1992. Schumer, for instance, quickly agreed with his more junior colleagues and joined Majority Leader Mitch McConnell, R-Ky., in calling for an ethics investigation of Franken.
"Sexual harassment is never acceptable and must not be tolerated," Schumer said in a statement, calling it a "troubling incident" and a "credible allegation."
No Senate Democrat has called for Franken's resignation, but that is a step that they are likely to consider if more allegations emerge. Gillibrand, in an interview with the New York Times, joined a small but influential chorus of liberals who now say that Bill Clinton should have resigned the presidency in the late 1990s following his affair with Monica Lewinsky.
Franken has pledged full cooperation with the investigation and has sent a personal apology to Leeann Tweeden, now a Los Angeles radio broadcaster who read the entreaty Friday on "The View."
In any previous political fight, Franken could lean on Schumer for advice and counsel. Now, in a testament to the precarious politics of the moment, Schumer must remain publicly neutral and careful about any private advice he gives. It is a remarkable shift in their relationship.
For years, Schumer would tear up when he recounted what he considered the best ad of 2008, a powerful testimonial from Franken's wife, Franni, on how her husband rescued her during her battle with alcoholism.
As chairman of the Democratic Senatorial Campaign Committee for four years, Schumer grew the caucus from 45 to 60 seats. But that last seat was the most cherished.
Up against then-Sen. Norm Coleman, Franken stumbled and struggled with allegations of sexism in his writing as a comedian and author. Finally, Schumer dispatched two future stars to Minnesota to help take over the drifting effort: Stephanie Schriock, who is now president of Emily's List, and Eric Schultz, who went on to serve as President Barack Obama's deputy press secretary.
That fall, Obama had pulled well ahead of Sen. John McCain, R-Ariz., in the presidential race. The biggest question wasn't whether Schumer would hold onto the Senate majority or even gain seats; it was whether he would win enough to hit the magic number of 60.
A few weeks after Election Day, every race but one had been called, and Democrats held 58 seats. Then Arlen Specter of Pennsylvania switched parties. And finally, on June 30, 2009, the Minnesota Supreme Court declared Franken the winner in the prolonged Minnesota recount. He was sworn in with great fanfare a few days later and, after his first policy luncheon with Democrats, Schumer proudly emerged to tell reporters how serious and thoughtful the former comic was in talking to his new colleagues.
Franken quickly fell in line with the more liberal wing of his caucus, but he became a quick supporter of Schumer's march up the leadership ranks. That helped provide cover for the New Yorker with the more progressive wing of the caucus and the activist base, whose trust in Schumer has been tenuous given his long ties to Wall Street donors.
Before Schumer became the Democratic leader, Franken had given him a nickname that made clear where he thought Schumer's rise would end, likening him to the most powerful majority leader ever. "I call him the Jewish LBJ," Franken told The Washington Post in an interview last year.
This was supposed to be Franken's breakout year. His close friend took over the caucus, and the former comedian published a new book, "Giant of the Senate."
He had started to appear on more Sunday political talk shows and sat for more profiles in the national media, using his wit and celebrity image to serve as a counter to Trump's own celebrity.
Some had even whispered that Franken would be the right fit to run against Trump in 2020.
"I really like this job. I like representing the people of Minnesota," he told The Post in the spring. "I feel like I'm really beginning to know this job."
Now, Franken has a new fight on his hands to keep this job - and it may be out of his longtime ally's hands. "I hope and expect that the Ethics Committee will fully investigate this troubling incident," Schumer said, "as they should with any credible allegation of sexual harassment."