What Polling Can Tell Us About Impeachment (and What It Can’t)

A majority of Americans have long opposed impeaching President Trump. Whether that will change, and what happens if it doesn’t, are some of many factors that will shape the political consequences for both parties.

The risk to Democrats had been straightforward: With impeachment so unpopular, pursuing it would turn the nation’s attention to a rare issue in which they would be taking a position even less popular than the president himself. It might polarize the electorate’s views of Congress along the same lines, endangering the many Democrats who represent districts carried by Mr. Trump.

But with Speaker Nancy Pelosi’s decision on Tuesday to seek impeachment, there is at least the potential for public opinion to start to shift.

There isn’t much polling on the Ukraine-related allegations, which have unfolded rapidly over the last few days. And it will take several days before any high-quality surveys are conducted in the aftermath of Ms. Pelosi’s announcement.

Even when those polls are released, they will yield only a glimpse of where voters stand at the beginning of what will be a long process. As more facts come out, public opinion is likely to evolve.

Until now, polls have shown Americans opposing impeachment by a fairly wide margin: 56 percent to 39 percent, according to data collected by FiveThirtyEight.

These polls were taken when the case for impeachment would have focused mainly on the Russia investigation and allegations of obstruction of justice, so they may not prove relevant to the new allegations about the president. But they represent the starting point for public opinion, and on paper, they would pose a substantial political risk to Democrats.

Whether that risk translates to actual damage depends in part on why so many voters have opposed impeachment, and whether that might change.

There is certainly upside for the Democrats, at least from their low starting mark (39 percent). A substantial share of opposition comes from self-identified Democrats, and it is reasonable to expect that many of these skeptics will come to their party’s side over the coming weeks.

Some of these voters may support impeachment on the merits but have questioned whether Congress should pursue it if it is unpopular or unlikely to result in Mr. Trump’s removal from office. Others might have assumed that the case for impeachment was not strong, given that the party’s elected officials have generally not supported it. They might see it differently now that support from congressional Democrats is all but unanimous.

Alone, these Democratic-leaning voters could move the polls to something close to majority support for impeachment. At best for Democrats, perhaps these voters by themselves could bring the polls to something near where they were in the Supreme Court nomination of Brett Kavanaugh when a slight majority of voters opposed his confirmation.

It is also possible that these easy Democratic targets would leave impeachment well short of majority support. So long as that’s true, there may be some political upside for Republicans.

Whether Democrats can move support higher will probably depend on the allegations themselves. Most voters probably have a very high threshold for supporting impeachment and removal from office. They might think it requires conclusive evidence of criminal activity. Even that may be insufficient, as was essentially the case for President Bill Clinton in 1998.

It is hard to know whether the Ukraine allegations will cross the threshold for a majority of voters. One piece of evidence suggests that it might: A YouGov poll, conducted on Monday, found that 55 percent would support impeachment if “President Donald Trump suspended military aid to Ukraine in order to incentivize the country’s officials to investigate” Joe Biden and his son.

Of course, it remains to be seen whether the hypothetical situation laid out in the poll will prove to be true, and persuasive to most voters. The president, along with many Republican allies, will argue it is an insufficient basis for impeachment. They will probably convince some number of Mr. Trump’s supporters of that.

But the poll at least suggests that the allegations have the potential to move public support for impeachment.

The facts of the president’s conduct, of course, will not be the only factor shaping public opinion. The behavior of congressional Republicans will be crucial, perhaps especially if a modest number either support impeachment or signal the seriousness of the allegations. A strongly unified Republican Party might be able to damage Democrats for pursuing impeachment, but a divided party might not be able to do so.

Of course, the conduct of Republicans and public opinion are not independent of each other. Mr. Trump’s standing could encourage or dissuade Republicans from taking his side, which would in turn help or hurt the president at the polls.

The clearest indicator of the politics of the impeachment proceedings might be the president’s approval rating. Today, it stands at around 44 percent of registered voters, according to the FiveThirtyEight tracker.

In the past, the rating has been somewhat responsive to events: President Trump’s decision to fire the F.B.I. director James Comey quickly knocked about three points off his approval rating, for example. His lows have tended to come during periods when the news was focused on unpopular initiatives that yielded intra-Republican fights, as when the Senate rejected the health care bill or in the run-up to the passage of the tax law.

Attitudes about the president are so entrenched that it is not realistic to expect an enormous swing in his approval rating. But if it is largely unmoved over the next few months, it will suggest that Republicans could be positioned to exact a political cost against the Democrats.

If his rating slump, the question will be whether an eventual acquittal in the Senate would be enough to undo the damage or more. That is very difficult to predict, which can be said for the whole impeachment process as it gets underway.

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