President Biden’s approval rating has taken a dip in recent weeks — but it’s not even close to the drop in support for Congress’s performance, as negotiations over legislation in Washington have ground to a virtual standstill.

That’s the top-line takeaway from a national poll released on Wednesday by Monmouth University. But here’s the secondary message: Democrats are the ones growing most disillusioned, and fast.

Back in April, when Mr. Biden was making big legislative strides, 83 percent of Democrats said they thought the country was moving in the right direction, according to a Monmouth survey at the time. But in Wednesday’s poll, just 59 percent of Democrats said that.

The share of Democrats saying the country was on the wrong track rose by 20 percentage points, to 32 percent.

“People are anxious — and look, Biden had such success at the outset with the Covid relief package that it probably got people’s expectations up very high about how much could be done and how soon,” Bob Shrum, the director of the Center for the Political Future at the University of Southern California, said in an interview. “Now reality is intruding.”

The $1.9 trillion economic relief bill that Mr. Biden signed in March remains broadly popular, with six in 10 Americans expressing a positive opinion of it, according to the poll. That’s basically on par with the 63 percent who gave it positive marks in a Monmouth survey in April, soon after the bill was passed.

And support for the president’s other top priorities remains high. Sixty-eight percent of Americans support the American Jobs Plan, his $2 trillion infrastructure plan, and 61 percent back the American Families Plan, his similarly large-scale proposal to invest in health care, child care and education.

But that support isn’t reflected in Congress, where Mr. Biden’s party holds the barest of control of both chambers — and where even some Democratic lawmakers haven’t fully gotten behind his proposals. Senators Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona have insisted on finding bipartisan compromise, rather than passing those two plans through the process of budgetary reconciliation, which would allow the bills to become law without any Republican votes.

The resulting standstill in Washington has left many Democrats feeling impatient. In the April poll, 63 percent of Democratic respondents said they approved of the job that Congress was doing. But that number has now been cut in half: Just 32 percent of Democrats gave Congress positive marks in the Monmouth survey released Wednesday.

Approval has also fallen among independents, though it didn’t have as far to go: It fell to 13 percent in the new poll from 28 percent in April.

Among all Americans, approval of Congress was down to 21 percent from 35 percent in April. (Though still paltry, that April figure had represented Congress’s highest approval rating at any point since 2013, when Monmouth started asking the question.)

“All in all, these poll numbers are still pretty good, given how polarized the environment is — but there’s no denying that some in the base want to see more accomplishments than we’ve had so far,” Jim Manley, a veteran Democratic strategist, said in an interview.

“I think the moment demands as aggressive an approach as possible, but the reality is, nothing is coming easy on Capitol Hill, and not everything Democrats want is going to get done,” he said.

The president’s approval rating has dropped by six points since April, and sits at 48 percent in the latest poll. This is the first Monmouth survey since Mr. Biden took office in which his approval has fallen below 50 percent. Still, with just 43 percent expressing disapproval, he isn’t in the red.

The dip was driven partly by liberals, whose approval of Mr. Biden fell by 12 points, to 76 percent, in the latest poll.

Mr. Manchin, a Democrat representing a deeply red state, voted for the widely popular relief package in March, but he has staunchly resisted passing another round of legislation without Republican buy-in. And he has been equally unwavering in his refusal to nix or weaken the filibuster, as many Democratic leaders and progressives have said they would like to do.

He is currently at work with Ms. Sinema and a group of Republican senators on a compromise infrastructure bill. Those talks have yet to result in a viable counterproposal. Some progressive Democrats have indicated that they would back a compromise on infrastructure only if Mr. Manchin and Ms. Sinema pledged to support passing a version of the American Families Plan through reconciliation.

While Wednesday’s Monmouth poll found the public to be broadly supportive of both the American Jobs Plan and the American Families Plan, respondents were more divided on how Democrats ought to pass them.

Forty-six percent of the country said that Mr. Biden’s major proposals should be passed as is, even if they didn’t get any Republican support, while 22 percent said they would favor significantly cutting the proposals down in order to gain bipartisan backing. Twenty-four percent said the bills shouldn’t be passed at all.

Among Democrats and liberals, there was little ambiguity: Four in five respondents in those groups said that Mr. Biden’s bills should be passed in their original form, no matter the Republican opposition.

Patrick Murray, the director of polling at Monmouth, pointed to another question on the survey as an important litmus test: Respondents were asked whether they thought Mr. Biden’s policies had benefited the middle class. Fifty-one percent said they had done at least a little to help, including three-quarters of Democrats, according to the poll. But just 19 percent said the policies had made a of difference.

Mr. Murray said that as Democrats looked ahead to a high-stakes midterm election cycle next year, voters’ feelings about how Mr. Biden’s bills have affected them personally would matter more than how he passed them.

“At the end of the day it’s about the results, and not how you got there,” Mr. Murray said in an interview. “So if you’re looking at 2022, what you should be looking at is: What’s going to be passed by then, and how are people going to feel it’s benefited them personally?”