WASHINGTON — Consumer confidence slumped last week to the lowest level in a month as Americans prepared to lose a portion of their pay to taxes.
The Bloomberg Consumer Comfort Index fell to minus 34.4 in the seven days ended Jan. 6, from minus 31.8, for the biggest one-week drop since August. All three components of the measure declined.
Paychecks will shrink because Congress agreed to let the payroll tax that funds Social Security benefits revert to 6.2 percent, up from a temporary 4.2 percent rate. Americans will have to rely on increases in salaries to counter some of the lost income.
‘‘Policy tensions surrounding the budget standoff in Washington and the reset of the payroll tax were the likely catalysts for a decline in consumer comfort,’’ said Joseph Brusuelas, senior economist at Bloomberg LP in New York. ‘‘Most households were probably blissfully unaware that their after-tax income would fall as a result of the partial solution to the fiscal cliff.’’
The decline in comfort was broad-based. The personal finances gauge fell to minus-2.6 from 0.8 the prior week, the first positive reading since July. The barometer that measures views on the state of the economy decreased to minus 58.7, from minus 57.1. The buying climate index dropped to minus 41.9, falling for a third consecutive week to reach the weakest level in more than three months.
The overall comfort index was still better than the 2012 average of minus 38.1. Last week’s decrease was within the margin of error of 3 percentage points.
Other sentiment measures showed Americans’ moods had soured before they knew lawmakers would allow taxes to rise. The Thomson Reuters/University of Michigan confidence index fell to a five-month low in December. The decrease was the biggest in more than a year. The Conference Board’s index fell last month by the most since early 2011.
Congress agreed Jan. 1 to let a payroll tax cut expire, which means an individual taking home $50,000 annually will lose a little more than $80 a month. The levy will probably have a larger impact on lower-income Americans.
The Bloomberg index indicated those with less disposable income showed the biggest declines in confidence last week. Americans making between $25,000 and $39,999 were the most pessimistic in three months, and confidence among those earning $15,000 to $24,999 a year fell by the most in two months.
Consumers on the other end of the income scale remained more confident. Thursday’s figures showed comfort among those earning $50,000 or more annually climbed to the highest level since November 2007. Sentiment among those making $100,000 or more was the highest since October 2010.
The fight over the federal budget also affected sentiment along partisan lines. The confidence gap between Republicans and the more optimistic Democrats was the widest in records going back to 1990.
The Consumer Comfort Index is based on telephone surveys from a random sample of 1,000 consumers 18 years old and older. Each week, 250 respondents are asked for their views on the economy, personal finances, and buying climate. The percentage of negative responses is subtracted from the share of positive views and divided by three. The most recent reading is based on the average of responses over the previous four weeks.
The comfort index can range from 100 — indicating every participant in the survey had a positive response to all three components — to minus 100, signaling all views were negative.