Senate Democrats on Tuesday will hold a critical test vote on a bill to curb a Supreme Court campaign finance ruling opposed by the Obama administration, but they appear to be one vote short of the 60 needed to pass the measure.

Despite what appeared to be a looming defeat, President Obama on Monday pushed for the bill's passage in a Rose Garden speech, saying a vote to oppose it "is nothing less than a vote to allow corporate and special interest takeovers of our elections."

Republicans think the bill would give an advantage to the Democratic Party as the critical midterm elections approach by blocking or discouraging donors more likely to give to GOP candidates.

Senate Minority Leader Mitch McConnell, R-Ky., said Monday the bill "seeks to protect unpopular Democrat politicians by silencing their critics and exempting their campaign supporters from an all-out attack on the First Amendment."

The bill would modify the Supreme Court's decision in Citizens United v. Federal Election Commission, which found that the government cannot impose restrictions on corporations' political donations any more than it can restrict giving by individuals.

The court handed down the ruling in late January. A few days later, Obama excoriated the court over the 5-4 decision during his first State of the Union address, as many of the justices looked on from the well of the House. Justice Samuel Alito mouthed the words "not true."

Months later, House and Senate Democrats crafted their legislative response to the ruling.

The bill is authored by Rep. Chris Van Hollen, D-Md., and Sen. Charles Schumer, D-N.Y., who head their party's campaign arms.

The legislation would require organizations to reveal the names of large donors and include those names in political advertisements. It would also block campaign funding from any company owned by a foreign government or in receipt of federal funding. Any corporation that accepted money from the $787 billion government bailout would also be banned from funding campaigns.

The bill passed the House in June, at which time Democrats were forced to carve out exemptions for large, established nonprofits, including the Sierra Club and the National Rifle Association, in order to bring moderates in their caucus on board.

In the Senate, Democrats control just 59 votes. So far, no Republican is willing to back it, with some saying they would rather focus on legislation aimed at boosting jobs and the economy. Democrats have tried to lure moderates such as Sens. Olympia Snowe and Susan Collins, both Maine Republicans, by stripping some pro-labor exemptions from the House version.

Sean Parnell, president of the conservative Center for Competitive Politics, called the bill "a frontal assault on First Amendment rights" that tips the balance in favor of unions over corporations, despite the modifications made by the House.

But Lisa Gilbert, a top official with the public advocacy nonprofit U.S. Public Interest Research Group, said the bill does not favor any group. "The goal of this bill is merely to take the blinders off because in the wake of the Citizens United ruling there is infinitely more money in the game."