Members of Congress have the benefit of relatively uniform and predictable public opinion in their districts. Presidential candidates don't have that, especially in the general election.

There's also the issue that they generally want to be talking about "policy" at this point. And then there's the issue that they're talking to Democrats, and everybody agrees that they hate Trump, so there isn't much to gain.

But it seems like quite a few have come out in support of impeachment over the past month. I haven't been paying attention, but I think Warren, Harris, Booker, Castro, I THINK Sanders but I'm not sure. I think Gabbard is the only one who's expressed opposition.