Senators Clinton & Obama are heading back to Washington D.C. today to vote on an economic stimulus bill.

The measure needs 60 votes to pass and right now the outcome is too close too call, according to senators and aides on both sides of the aisle.

However, heavy lobbying from the AARP and other interest groups has raised Democratic hopes. One Republican senator who opposes the Democrats’ bill — John Thune of South Dakota — told CNN the lobbying had forced Republicans to change their strategy on the issue, although most still oppose the bill.

Until Tuesday, Republican leaders pushed for an up-or-down vote on the House-passed bill, which is centered on rebate checks for low- and moderate-income taxpayers. But after the lobbying intensified, GOP leaders said they would support providing rebate checks to more people — agreeing with Senate Democrats that low-income Social Security recipients and disabled veterans should get rebates.

But most Republicans still adamantly disagree with other parts of the Democratic bill, such as the inclusion of new unemployment benefits and assistance for energy costs for low-income households.