Yet all of us should have learned by now that there is a big difference between the unlikely and the impossible. Political polls are like weather forecasts or medical diagnoses. They are a much better guide to the future than random guesses, but they inevitably involve uncertainty. “84 percent” is not just a careful way of saying “100 percent.” The Republican Party of Donald Trump really could win the midterms.

That’s why I spent a good portion of this past week doing reporting about what two more years of unified Republican rule might look like. The following preview is based on those conversations.

The Russia investigation

Members of Congress don’t usually turn against a scandal-marred president from their own party until they believe that supporting him jeopardizes their own careers. That fear helped lead Republicans to abandon Richard Nixon, at long last, in 1974. A lack of fear helps explain why Democrats stuck by Bill Clinton through his sex scandal.

For today’s congressional Republicans, the election will be a measure of how worried they need to be about Trump and Russia. As Susan Hennessey of the Brookings Institution says, the midterms “are the last best hope for Republicans in Congress to grow a backbone.” A Democratic takeover may make congressional Republicans less willing to make excuses for Trump. A Republican victory would suggest that the party does not need to be very scared about the politics of Russia.

Yes, Mueller — the special counsel overseeing the investigation — could change that by issuing an explosive report after the election. But a midterm victory could also give Trump the confidence to shut down the investigation, through firing some combination of Mueller, Rod Rosenstein (who oversees Mueller) and Jeff Sessions, the attorney general. In their place, he could install loyalists unconcerned with the quaint notion that no one is above the law. An expanded Senate majority could ease the confirmation of those loyalists.

Even if Mueller’s findings eventually became public, an unfinished report is different from a completed investigation that could include indictments of people close to Trump. It’s easy to imagine Trump waving it away as a “witch hunt.” Congressional Republicans may go even further and hold hearings investigating the investigators — the current and former law-enforcement officials whom Trump has unjustly blamed for the investigation.

Altogether, an election victory would probably give Trump the confidence to do what he has already signaled he wants to do: use the law as an instrument of political power rather than one of justice.