U.S. President Barack Obama sought on Saturday to boost pressure on Republican lawmakers to back an extension of a tax cut for workers that he views as vital to help the fragile economy.

Obama's proposal to renew a temporary payroll tax holiday has received a lukewarm reception from Republicans who say it would not do much to spur economic growth and would weaken the Social Security retirement program.

Without congressional action by the end of the year, the payroll tax would revert to 6.2 percent from the current rate of 4.2 percent.

Obama has warned that failure to extend the cut would deal a huge blow to the economy.

Now is the time to step on the gas, not slam on the brakes, Obama said in his weekly radio and Internet address. Unfortunately, too many Republicans in Congress don't seem to share that same sense of urgency.

In an attempt to rally help for his effort, Obama urged Americans to visit the White House Web site to calculate how their household incomes would be affected by the expiration of the payroll-tax cut.

Try it out. Then let your members of Congress know where you stand, Obama said, adding that the average family would see their tax bills go up by $1,000 if the tax cut lapses.

The radio address was the latest in a series of public remarks Obama has been making to try to boost support for the payroll tax cut.

On Thursday, the Senate defeated competing versions of legislation that would have extended the payroll-tax cut. The vote exposed divisions among Republicans as many declined to back a version of the bill put forth by their own leadership.

While some Republicans are unenthusiastic about the payroll-tax cut, others worry they could suffer fallout in congressional and presidential elections next November if they are seen as blocking a tax reduction for middle-class Americans.

Obama is grappling with weak approval ratings because of the lackluster economy and high unemployment. Any improvement in the economy would help his chances of winning another four-year term in office.

On Friday, the U.S. Labor Department reported the unemployment rate fell to 8.6 percent in November from 9 percent in October, offering a hopeful sign for the economy. But employers added workers at a modest pace of 120,000, suggesting the labor market still lacked vigor.

After gaining little traction for his efforts to pass in full the $447 billion jobs he unveiled in September, Obama is making an aggressive push on parts of the bill such as the payroll tax cut and an extension of jobless benefits that he sees as having the greatest chance of passage.

(Writing by Caren Bohan; Editing by Chris Wilson)