WASHINGTON (CNN) — Senate Democratic leaders have summoned presidential candidates Sens. Hillary Clinton, D-N.Y., and Barack Obama, D-Ill., to the Senate Wednesday for a 5:45 p.m. vote on a Democratic-backed 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.

Republicans want to offer their own amendment but so far have been blocked by Democrats.

Democrats say they need at least nine Republicans to vote with them for the measure to pass. Several GOP senators, such as moderates like Olympia Snowe of Maine and Gordon Smith of Oregon, and one senator up for re-election — Elizabeth Dole of North Carolina — have said they will vote with the Democrats.

Attempting to force the hands of wavering Republicans, Senate Majority Leader Harry Reid, D-Nev., has suggested he won't allow a vote on anything but the Finance Committee bill and the stand-alone House-passed bill. That could mean that any Republican who votes against the Democratic bill risks being accused of voting against seniors and veterans. Democrats acknowledge they've created an uncomfortable circumstance for many Republicans.

That said, there is also a growing group of GOP senators who think a stimulative package based on rebates is not good policy and the whole thing should be scrapped.

Republican presidential hopeful Sen. John McCain of Arizona is expected to return for the vote, but it could not be determined if he'll vote for or against the bill.

– CNN Congressional Producer Ted Barrett