It would create a government "car czar" to dole out the loans, with the power to force the carmakers into bankruptcy if they didn't cut quick deals with labor unions, creditors and others to restructure their businesses and become viable.
Congressional Republicans, left out of negotiations on the package, are expressing grave reservations and may seek to block it.
Sen. David Vitter, R-La., promised to filibuster the measure, which could delay a final vote for days.
He said the package has an "ass-backwards" approach to curing what ails the U.S. auto industry.
Nevertheless, Democratic leaders were confident enough that a bill could advance that they set a procedural vote for the House floor later Wednesday. Even still, Sen. Mitch McConnell, the GOP leader, said in late morning that his side hadn't seen the measure yet and wouldn't agree to votes on the measure Wednesday.
"Republicans will not allow taxpayers to subsidize failure," McConnell said, although he added that the auto situation would be addressed by the end of the week.
The congressional officials revealed agreement on a bill only on grounds of anonymity because the deal has not been formally announced.