A new Monmouth University poll found 40% of Americans approve of the GOP tax law, while 44% disapprove.

Perhaps the more troubling number for Republicans: Only 38% of Republican voters strongly support the law, down from 49% in January.

The GOP is already facing an enthusiasm gap in the 2018 midterms, so the eroding support for the party's signature achievement is a worrying signal.

The signature legislative achievement of President Donald Trump's presidency remains unpopular, according to a new poll. In fact, even the GOP's own adherents are starting to cool on the measure.

A new Monmouth University poll found that 40% of Americans support the Republican tax law, while 44% disapproved. That represented a slight shift from the 44% who approved and 44% who disapproved in a January poll.

Perhaps most troubling for the GOP, however, is that Republican enthusiasm for the law appears to be waning.

According to the Monmouth poll, 38% of Republicans strongly approved of the law. That was from 49% who strongly approved in January.

Most of those GOP voters who no longer strongly approve still generally approve of the law. And disapproval by Republican voters remained low, up slightly to 17% in May from 9% in a March poll.

But in an election in which Republicans are already facing a significant enthusiasm gap, the waning fervor could be a concerning sign for the party. Republicans are expected to campaign heavily on the tax law. As a whole, respondents in the poll were more likely to strongly disapprove than strongly approve.

"There is now an 11-point gap between those who strongly approve (18%) and strongly disapprove (29%), which is somewhat larger than the 6- to 7-point gap in prior polls," the poll found.

Other polling has also shown the tax law to be overall unpopular. A Gallup poll in April found that 39% of Americans approved of the law, while 52% disapproved. Another poll from NBC News and The Wall Street Journal found 27% of people thought the tax law was a good idea, while 36% said it was a bad idea.