WASHINGTON — The Senate is poised today to add $2 billion to the popular "cash-for-clunkers" program after lawmakers agreed to vote on the government car incentives and give shoppers until Labor Day to visit dealerships and make a deal.

Administration officials have estimated the tripling of the $1 billion program could fund an additional 500,000 new car sales, giving automakers a late summer boost after months of ragged sales. The Obama administration has said the program would go broke by Friday without congressional approval of the extension.

Wrapping up lengthy talks, Senate Majority Leader Harry Reid said Democrats and Republicans had agreed to vote on the plan today while considering a series of potential changes to the bill, which was passed by the House last week. Senate passage would send the legislation to the White House for President Barack Obama's signature and assure consumers there will be no interruption in the program that has led to packed car dealerships nationwide.

Reid has said Democrats have enough votes to approve the measure and beat back any changes that would lead to an interruption in the rebates of $3,500 or $4,500 for trading in gas-guzzlers for new, higher-mileage models.

None of the proposed amendments was expected to pass. They included placing an income limit on those benefiting from the vouchers, terminating the Troubled Asset Relief Program and requiring the government to sell off its shares in Michigan automakers General Motors and Chrysler Group.