Yea I've heard this narrative before almost word for word. Republicans were a minority party in both the House and the Senate during Nixon's impeachment, and only 13 would need to flip to convict Nixon. It was also 1973, and there were Republican senators in New York, New Jersey, Pennsylvania, Vermont, Virginia, New Hampshire, Connecticut, Massachusetts, Oregon, Illinois, Colorado, and Michigan. A year later there were even fewer Republicans in both houses (7 senators, 34 congressmen).
You might swing a general population 2-5% more than the 47% they're at now, but it doesn't matter on a state by state basis. You're certainly not going to sway them with hearsay from anonymous Democrat insiders with an opinion. It's also interesting that you seem to approach this from the possibility that support for removal can only go up.
If you'd like to take a shot though, I'd love to hear which 20 Republican senators are in seats so competitive that a 5-10% increase in support for removal nationwide would make them more afraid than they would be of the Republican voter backlash locally.
Yeah, times are definitely different in all sorts of ways. I'm gonna glance at the list and unscientifically see how many I think could possibly flip due to various forms of political pressure:
Lamar Alexander (retiring)
Richard Burr (has shown some objectivity in Trump-related matters)
Shelley Moore Capito (somewhat moderate from what I know, up for reelection)
Susan Collins (swing state, up for reelection)
Ted Cruz (doubtful but you know he hates Trump deep down)
Mike Enzi (retiring)
Joni Ernst (politically-engaged swing state, up for reelection)
Cory Gardner (swing state, up for reelection)
Lindsay Graham (personally hates Trump, up for reelection, has changed positions a zillion times)
Chuck Grassley (probably not but same state as Joni Ernst and has shown some objectivity)
Ron Johnson (swing state, dude can't figure out what he believes)
Mitch McConnell (hates Trump, up for reelection, could decide he's better off with Pence and bring the caucus with him)
Martha McSally (swing state)
Lisa Murkowski (vulnerable and sometimes breaks with GOP)
Rob Portman (politically-engaged swing state)
Pat Roberts (retiring)
Mitt Romney (duh)
Marco Rubio (hates Trump, young and has eyes on the future)
Ben Sasse (young, sometimes speaks against GOP but rarely acts, up for reelection)
Pat Toomey (swing state)
I went down the list without counting and that is coincidentally 20 GOP senators who I could see flipping in bizarro-world. The wild card is, of course, McConnell. If he decides it's time to send Trump upstate, many others would follow in his footsteps. This is a 10-20% chance in my estimates, but it's closer to 10% than it is to 20%.