Michael Cohen didn’t push the story forward very much in his testimony to the House Oversight Committee yesterday morning, although there were some interesting hints about what federal prosecutors in New York might still be looking into.

But he did confirm a lot of what people paying close attention already believed — in particular that President Donald Trump is (as Cohen put it) a “con man” and a “cheat” who frequently directed his personal lawyer and others to do unethical and even illegal actions.

Will any of it make a difference? For the most part, it’s likely that the hearing will just confirm the biases of most solid partisans: Democrats will hear that Trump is a crook; Republicans will hear that Cohen is a convicted liar who can’t be trusted.

Still, there could be some immediate and long-term effects on public opinion.

For one thing, the news media treated his hearing as a big deal: not only did the cable networks cover it live, but the broadcast networks (ABC, NBC, CBS and Fox) did, too.

The coverage sent a strong signal that something important was going on, which means that a lot of people who don’t normally pay much attention to politics between elections will take notice and probably learn one or two headline-type facts.

And to casual viewers who are not strong partisans, the hearings presented a very bad narrative about Trump.

Stories about hush-money payoffs, cover-ups and lies told by the President and the President’s men.

These are stories that those following the news know well, but that others may have just ignored.

And those folks who don’t follow the news very closely are far less likely to have fixed views of Trump or anything else in politics.

Those within the relatively small group who actually pay a lot of attention to politics and public affairs may find it hard to believe that anyone hasn’t made up their mind about the President long ago.

And yet Trump’s approval rating dropped almost four percentage points last month because of a blast of bad news about him.

It recovered (it’s currently in the low 40s) when the bad news disappeared.

Four points may not seem like a lot, but if Trump improved by that much he’d be back to looking like a toss-up for re-election in 2020 — and if he lost four percentage points he would look not even competitive.

Overall, my guess is that Cohen isn’t a particularly compelling witness. And he’s actually testified that some of the wilder rumours about Trump are, as far as he knows, not true.

But his basic testimony, backed by documents and consistent with plenty of reporting, seems to hold up pretty well.

Meanwhile, Republicans on the committee aren’t helping their own cause very much.

They’ve repeatedly questioned whether he can be trusted, but haven’t actually discredited the basic things he’s saying.

And they have an inherent problem: the worse Cohen looks, the worse Trump looks for relying on him for ten years.

• Jonathan Bernstein is a Bloomberg Opinion columnist covering politics and policy