WASHINGTON - None of the 20-plus Democrats running for president have wanted to make impeaching President Trump a central focus of their campaign. That is, until last week when billionaire businessman and impeachment advocate Tom Steyer entered the race.

But the latest NBC News/Wall Street Journal poll shows enthusiasm for impeachment may be waning and that could leave the Democrats’ 2020 hopefuls with a voter base that sees things differently and a complicated issue to tackle as their primary campaign revs up.

Overall, the latest NBC News/Wall Street Journal poll shows a decline compared to last month in the number of people who say they want to hold impeachment hearings now.

The latest NBC News/Wall Street Journal poll shows a decline compared to last month in the number of people who say they want to hold impeachment hearings now.

The latest survey finds that 21 percent of registered voters say that there is enough evidence for Congress to begin impeachment hearings now. In June, 27 percent in the poll the same thing, a six-point drop in one month – though that survey was of Americans, not registered voters.

The survey isn’t all good news for Trump. The latest survey showed a three-point increase in the number of people who want to see Congress continue its investigation and see where it leads.

And there has been little change in the number of people who want to see Congress drop the impeachment inquiry and let the president complete his term. That number was at 50 percent in this poll, largely unchanged since 48 percent last month and 47 percent in March.

It doesn’t take much imagination to guess at which voters are driving the nation’s pro-impeachment sentiment.

It doesn't take much imagination to guess at which voters are driving the nation's pro-impeachment sentiment.

Nearly 40 percent of Democrats say there is enough evidence to hold impeachment hearings right now. Independents are quite a bit lower on the question, with a 21 percent number that mirrors the national overall figure. And Republicans want nothing to do with the topic, 85 percent say they want impeachment dropped.

So if Democrats are more excited about impeachment than the public at large, how will the 2020 hopefuls respond when former Special Counsel Robert Mueller testifies before Congress in the coming days? That could depend a lot on the candidate.

Supporters of front-runner Joe Biden are a little less interested that most Democrats in moving impeachment forward quickly.

Supporters of front-runner Joe Biden are a little less interested than most Democrats in moving impeachment forward quickly, 37 percent of his supporters say they believe there is evidence to begin proceedings now.

In the top tier of Democratic hopefuls, the numbers are much higher for those who support Sens. Bernie Sanders, Elizabeth Warren and Kamala Harris. For each candidate, the “begin hearings now” figure is closer to 50 percent.

And among the top finishers in the poll, South Bend Mayor Pete Buttigieg is an outlier. Only 28 percent of his supporters believe there is enough evidence to go forward with impeachment hearings right now. A higher number of Buttigieg’s supporters, 32 percent, believe there should be no impeachment hearings and Trump should be able to finish his term in office.

Keep these numbers in mind when Mueller speaks before Congress in the days ahead, because they will likely affect how the candidates respond. Democrats view the impeachment story a little differently than the public as a whole, but even within that smaller Democratic pool, these candidates are going to be talking to specific audiences.

What do those audiences want to hear? How far do the candidates want to push the path to impeachment or how far do they want to walk away from it? Those questions are going to matter for the 2020 campaign.

In reality, where the candidates stand on Trump's impeachment has little to do with what they would do if they win the White House. If any of them actually becomes president, the issue will be moot.

But politically speaking, impeachment is not likely to go away because it is about the main topic of the presidential campaign, Donald Trump. The way the Democrats react to him is almost certainly going to be the prime driver of the primary contests – and the general election.