A "few" in this case being that only 3.5% of Trump voters in a new Washington Post poll say they would change their vote today, as Steve M points out over at his place.
According to this survey, Trump retains the passionate support of nearly his entire voting bloc. Remember that his most primal instinct is self-preservation -- does he really care whether Republicans suffer in the midterms or there's less security and prosperity in America and the world? Yes, the Gallup poll is bad for him -- but it may just be that angry anti-Trump voters are more willing to talk to pollsters these days than Trump backers, who tend to believe all mainstream media and polling outfits are deceitful.
I believe that Trump is historically unpopular for a president in the first months of his term. But if he can sustain this level of support until 2020, he might just slip by a Democrat again, taking advantage of GOP voter suppression and a wealthy corporate donor community that will never bail on the GOP, not to mention campaign finance rules that are only going to get worse once Neil Gorsuch is on the Supreme Court.
Reminder: Trump won the election with about 25% of the total eligible voting population, because of the electoral college and the fact only about 26% voted for Hillary. If Trump's approval rating among voting eligible adults is 37%, and all of them vote for him, he'd win in a landslide.
People have to actually vote to beat Trump and the GOP, and so far not enough of us give a damn. So when Republicans shave off a couple percentage points with red state voter suppression tactics, it's a good move for them that will pay off for decades. It already gave them the White House and Congress (and soon the Supreme Court).
Remember that Richard Nixon and George W. Bush won reelection before disapproval of them reached critical mass. It takes a long time for Republicans to lose faith in one of their own, even when the rest of the country has already done so.
Credibility? Trump's base thinks he has all the credibility he needs, even now. And that base is essentially the entire GOP electorate.
The only way we win is a united front. As Zandardad said yesterday, we had the chance to unite behind Hillary and enough of us chose not to so that Trump won. We wouldn't be in this epic disaster documentary right now if we had just done the right thing. And before the usual suspects come in and complain, ask yourself given Trump's actions right now and his Supreme Court nominee, would you rather have Hillary or Donald as president today?
If you can't answer that, you're the reason why we have Donald. You picked a side, and it was Trump's. End of line.