Suddenly and all at once, a group of Senate Democrats are demanding their colleague Sen. Al Franken (D-Minn.) resign over multiple sexual harassment and groping allegations spanning a decade.

It's a big change from just weeks earlier, when they supported an ethics investigation into his conduct rather than asking him to resign.

“Be they political friends or foes, we must call out those who would seek to sweep under the rug behavior that should never be accepted in any family or community,” Sen. Patty Murray (D-Wash.), the top-ranking woman in Senate Democratic leadership, said in a statement.

Why are Franken's Democratic colleagues just now asking him to go? Franken has faced some of these allegations for three weeks, since Los Angeles broadcaster Leann Tweeden accused him of forcibly kissing her and said he “grabbed” her breasts while she slept.

Since then, Franken hasn't denied any of these allegations, save one that came out Wednesday morning. (More on that later.).

So, back to our original question: Why are Senate Democrats asking Franken to resign now?

The answer, as usual, comes down to politics. Franken's accusers kept coming, the culture is shifting on sexual harassment, and Senate Democrats risked being on the wrong side of history (and elections) if they didn't speak out now.

Let's break down the political dynamics that are probably playing a role in Franken's Senate colleagues turning on him:

The drip-drip-drip of Franken's accusers: Franken now faces sexual harassment allegations by more than a half-dozen women. On Wednesday, two more: Politico reported that a congressional staffer said she had to duck Franken when he tried to forcibly kiss her in 2006, just as he was ramping up for a Senate campaign. The accuser is anonymous, and Franken categorically denied this accusation.

Wednesday afternoon, Atlantic writer Tina Dupuy said Franken groped her in 2009. Franken didn't comment on that one.

Franken hasn't denied the six other accusations against him. And his defense — that he's a sloppy hugger — doesn't hold up against many of his accusers's stories. (Multiple women accuse him of groping them while they posed for a photo, not while they were hugging.)

Sen. Kirsten Gillibrand (D-N.Y.) indicated to reporters that this seventh accuser was the last straw. “There were new allegations today, and enough is enough,” she said.

Of course, Senate Democrats still need to answer why six, or five, or four, or three, or two, or one accuser weren't enough for them to say Franken should go.

Sen. Al Franken (D-Minn.) in July. (Pablo Martinez Monsivais/AP)

Which brings us to our next point ...

Most Washington Republicans have decided to back Roy Moore in Alabama: President Trump endorsed the Alabama GOP Senate nominee Monday, despite Moore being accused of sexual misconduct with teenagers when he was in his 30s. The Republican National Committee committed to spend money on him. Senate Republicans still don't want Moore in office, but they haven't committed to kicking him out.

Meanwhile, the only sitting members of Congress who have been accused of sexual harassment in this post-Harvey Weinstein era are Democrats. So, for the sake of their political future, Democrats may have realized they needed to force some of their own out to draw a moral distinction between their party and the other side.

The Time magazine award to the women of the #MeToo campaign: It's a safe assumption there are no coincidences in politics. Senate Democrats' decision to ditch Franken came just hours after Time magazine awarded its person-of-the-year cover to women from “virtually all corners of the globe” who shared their stories of sexual harassment post-Harvey Weinstein. As my colleague Philip Bump writes, the dissonance between this year's winners and last year's winner (President Trump, accused of sexual harassment by more than a dozen women) is hard not to notice.

Franken's first public accuser, Tweeden, is one of those who used the #MeToo hashtag. And the dissonance between the high-profile #MeToo Time magazine cover and Senate Democrats giving Franken the benefit of the doubt would have been conspicuous, too.

Some were introducing legislation to stop sexual harassment in the workplace: Literally minutes after Gillibrand became the first sitting Democratic senator to call on Franken to resign, she stepped up to a lectern in the Capitol and introduced legislation to make it easier for accusers to bring sexual harassment claims against their employers.

The optics of trying to change the culture of sexual harassment in the workplace while giving Franken the benefit of the doubt would have been terrible for Gillibrand and other supporters of the bill.

They got a playbook in Rep. John Conyers Jr.'s (D-Mich.) resignation: Congressional leaders have been hesitant to call out their colleagues to resign over sexual harassment allegations. Some of that hesitation is necessary, Kelly Dittmar, a women in politics expert at Rutgers University, said earlier this week: “There are protections in place to ensure popularly elected officials can't just get kicked out of office. The ultimate form of accountability in Congress, unlike the private sector, is being reelected or not.”

But Senate Democrats watched as, across the Capitol, House Minority Leader Nancy Pelosi (D-Calif.) said that Conyers, Congress's longest-serving member, should resign over sexual harassment allegations before he faced an ethics investigation. And on Tuesday he did, albeit without acknowledging wrongdoing.