Conventional wisdom holds that the House will impeach Donald Trump, but impeachment dies in the Senate. But as evidence for impeachment mounts (the recent developments about China and Syria are important), and as Trump continues to spiral out of control rhetorically, I’ve started believing there is at least a chance that 20 of the 53 Republican senators could vote to convict.

It’s easy to imagine the first defector. It’s Mitt Romney, of course, who has already been outspoken about Trump’s behavior. Breaking the seal, so to speak, is vital. As Michael Tomasky writes, “Romney… might embolden a few of his Senate colleagues whose desertion from the Trump ranks will make a difference. In fact, that may be a job, persuading other Republicans to speak out, that only he can do.”

Someone has to jump into the freezing swimming pool first, and that job probably belongs to Romney. Then, if it looks like the pool is warmer than suspected, others may take the plunge.

Among the most likely is Maine’s Susan Collins—whose past support for Brett Kavanaugh damaged her standing as a moderate. Collins recently said that Trump made a “big mistake” by asking China to investigate Joe Biden. She’s up for re-election in a state where Trump isn’t popular. As such, her joining Romney wouldn’t surprise people, and wouldn’t turn many heads.

For most Republicans up in 2020, though, the calculation is much dicier. If they appear to be too close to Trump, they can write off a lot of educated suburban voters. But if they vote to impeach, it will depress their rural base. For them (I’m thinking here of Colorado’s Cory Gardner, Thom Tillis of Tennessee, and Arizona’s Martha McSally) the decision will probably come down to a cost-benefit analysis that will be made as late in the game as possible.

Because these names constitute the senators who are obvious concerns for Trump, the more interesting ones (to me, at least) are the senators who might constitute a second or third wave of defectors. This got me thinking: Which five GOP senators might break ranks, and who would really matter?

Here are five names I’ll be watching:

1. Lamar Alexander of Tennessee. There are two reasons Alexander might vote to impeach: because it fits his personality; and he’s not running for re-election. As National Journal’s Josh Kraushaar put it concisely, “Alexander, who began his career working for Sen. Howard Baker on the Senate Watergate Committee, is an institutionalist who has more freely criticized the president since his retirement announcement.”

2. Lisa Murkowski of Alaska. Murkowski has essentially said that she wants to see how this all develops, but one factor makes her a wild card: She doesn’t need the Republican Party. Anyone who can lose a Republican primary only to go on to win a write-in campaign, as Murkowski did in 2010, doesn’t need to worry about ruffling partisan feathers. Winston Churchill’s line about being shot at with no effect rings true. Murkowski is a free agent, and that is both exhilarating and liberating.

3. Mike Lee of Utah. Lee’s commitment to the Founding Fathers and the Constitution makes him a threat to Trump on principle. Lee also benefits from the fact that he hails from Utah, a state that is culturally less Trumpy than most red states. If Mitt Romney votes to impeach (and if he doesn’t this is probably all a moot point) that would provide Lee, who is up for re-election—if he chooses to run again—with additional cover.

4. Jerry Moran of Kansas. He’s a moderate sort of guy. For now, Moran is critical of Nancy Pelosi’s “rush to judgment,” but things can change.

5. Rob Portman of Ohio. Like Moran, Portman is not currently for impeachment. Still, Portman has said, “The president should not have raised the Biden issue on that call, period. It’s not appropriate for a president to engage a foreign government in an investigation of a political opponent.” These are pretty strong words. Potrman has had a serious career, which includes serving as United States Trade Representative and Director of the Office of Management and Budget (OMB).

If this seems like a boring list, that’s not an accident. The 80/20 rule applies to the U.S. Senate, which is to say that 20 percent of the senators get 80 percent of the media attention (and vice versa). Lindsey Graham, for example, punches way above his weight. Someone like Chuck Grassley, although very powerful, doesn’t get nearly as much press attention, especially now that he’s handed the important Judiciary Committee off to Graham.

And it is when you start looking to some of these lesser-known names that you begin to unearth the potential for Republicans to hit double digits on impeachment. While the show horses tend to gravitate toward Trump, the workhorses tend to be less enamored.

If Jeff Flake is correct in saying that at least 35 senators would vote for impeachment if the vote was held in private, then it stands to reason that senators who are insulated from re-election concerns (not up for re-election in 2020, or safe in their incumbency) are more likely to demonstrate courage. What this list demonstrates is that this number is much bigger than you probably think.

“ Somebody has to go first and prove he or she can survive. ”

And I didn’t even mention names like Mike Enzi (who is retiring), Pat Toomey, or Roy Blunt—names that could potentially constitute a third wave of defectors.

Or how about Senate Intelligence Committee Chairman Richard Burr of North Carolina ?

This gets us to double digits.

Likewise, Marco Rubio and Ben Sasse have been excluded based on their recent political cowardice. But if the winds start to blow in a different direction, it’s possible their principle might reassert itself. Rubio, for example, is outspokenly opposed to Trump’s abandonment of the Kurds in Syria. And Sasse has recently been critical of Trump’s dealings with China.

If this happens, Trump has entered the danger zone, and the countdown goes from “Just 3 Republicans will vote to remove him” to “Just X More Republicans are needed…”

Of course, one of the biggest problems this whole gambit will confront is that Senate Majority Leader Mitch McConnell is not similarly liberated. For one thing, he’s on the ballot in 2020 (he’s already raising money by vowing to stop impeachment).

The bottom line is that getting 20 Republicans to vote for removal is a very high hurdle. On the other hand, more Republicans might be willing to vote for removal than most people imagine.

Somebody has to go first, and prove he or she can survive. That greases the skids, creates momentum, and emboldens other would-be heroes. There’s safety in numbers, too, which is why Trump will work hard to prevent this sort of momentum.

An idea: What if 10 of them all came out at once and acted as a group? That would be awesome, but this kind of thing only happens in movies. Probably.

Editor's Note: A previous version of this piece said that Jerry Moran is retiring. That error has been corrected.