President Trump speaks at a rally in Phoenix on Aug. 22. (Joshua Roberts/Reuters)

Gallup’s weekly assessment of President Trump’s approval rating hit a new low Monday. Over the preceding seven days, only 35 percent of the country approved of the job that Trump was doing — a decline of two percentage points from the week prior.

More important, though, it continues a general downward trend for Trump since his first week as president. In that first week, 45 percent of Americans approved of the job he was doing. He has since dropped 10 percentage points, more than a fifth of the support he had at the outset.

Of more concern to Trump, though, should be the erosion of support from voters who were critical to his election.

A lot of attention has been paid to the core, fervent base of support that Trump enjoyed from the primaries through the general election. But he won because more-moderate Republicans who were iffy about him ended up voting the party line. They, like a number of independents, didn’t really like him, but they liked Hillary Clinton even less.

That’s a problem because Trump has seen a big drop in support from precisely those groups. Republicans overall have shed 11 points of support for Trump — but moderate Republicans have dropped 17 points since January.

That group is smaller than the number of Republicans overall, meaning that the margin of error is bigger and the 17-point drop may be exaggerated. But when any group of Republicans has only a 58 percent approval of a Republican president, that’s … not great.

But this is simply not sustainable for a president who hopes to be reelected — and, perhaps, for his party if it hopes to maintain control of Congress in 2018.

Trump won last year despite, not because of, the amount of support he received in the election. He lost the popular vote by millions but won where it counted. His 78,000-vote margin in Wisconsin, Pennsylvania and Michigan was 0.06 percent of all votes cast, but it gave him the presidency.

This is one pollster, but it’s a pollster that polls regularly. If these numbers hold steady for 2020, Trump’s reelection is in deep trouble. If they continue to dip even lower, the party probably wouldn’t even want him on the ticket.