The government's emergency budget contained harsh measures to reduce Britain's record peacetime budget deficit, including a plan to raise the value added tax (VAT) paid on most goods and services to 20 percent from next year.
An ICM poll for the Sunday Telegraph showed Lib Dem support had fallen to 16 percent, down five points from before the budget and its lowest in 18 months.
By contrast, the Lib Dems' Conservative partners seem to be thriving, increasing their rating by two points to 41 percent, and Labour, which has criticized the budget's tough spending cuts, gained four points to 35 percent.
The left-leaning Lib Dems campaigned vigorously against a rise in VAT ahead of the May 6 election but had to compromise when they joined with the larger, center-right Conservatives to form Britain's first coalition government since World War II.
A YouGov/Brand Democracy survey in the Observer newspaper showed 48 percent of those who voted Lib Dem at last month's election were less inclined to back them again because of the VAT hike.
The poll was commissioned by Labour leadership hopeful Ed Miliband in a sign Britain's second-biggest party is turning up the heat on its smaller rival.
Although no high-level splits have emerged in the seven-week-old coalition, leaders on both sides have yet to convince skeptics their agreement is strong enough to last a full five-year term.
Writing in the Observer, Lib Dem chief secretary to the Treasury Danny Alexander admitted measures such as the VAT rise were proving unpopular with party members but insisted they were "unavoidable" given the state of the public finances.
Business Secretary Vince Cable, one of the most popular Lib Dem figures, took to the airwaves to highlight the compromises his party had obtained from the Conservatives, such as a rise in the level of income that is not subject to income tax.
"If you look at the package as a whole that makes the balance much better and more equitable," Cable told BBC television's Andrew Marr Show.
Cable dodged questions about whether he was now embarrassed by featuring in a Lib Dem poster campaign earlier this year which warned of a "Tory VAT bombshell."
"That was in the election. We have now moved past the election," he said.
