WASHINGTON (AP) — The Senate is on the cusp of approving historic immigration legislation offering citizenship to millions in the U.S. illegally and spending billions of dollars to secure the border.

The vote on final passage of the White House-backed bill was expected as early as Thursday, after a series of test votes so far this week demonstrated supporters command a bipartisan majority well over the 60 votes needed to secure passage and send the bill to the House. First must come two more procedural tests set for Thursday.

"We're on the edge of passing one of the most significant pieces of legislation that this body has passed in a very long time," Sen. Chuck Schumer, D-N.Y., said on the Senate floor Wednesday. "The vast majority of members in this body realize that the immigration system is broken and needs fixing."

Supporters posted 67 votes or more on each of three procedural tests Wednesday. More than a dozen Republicans sided with Democrats on each, ensuring bipartisan support that the bill's backers hope will change minds in the House.

The outlook there is uncertain. Many in the GOP-controlled House oppose the pathway to citizenship at the center of the Senate bill. And many prefer a piecemeal approach rather than a sweeping bill like the one the Senate is producing.

The House Judiciary Committee is in the midst of a piece-by-piece effort, signing off Wednesday on legislation to establish a system requiring all employers within two years to check their workers' legal status.

The Judiciary Committee was turning its attention Thursday to a bill on high-skilled workers. Last week it approved two more measures, one on agriculture workers and a second to make illegal presence in the country a federal crime, instead of a civil offense as it is now.

At its core, the legislation in the Senate includes numerous steps to prevent future illegal immigration, while at the same time it offers a chance at citizenship to the 11 million immigrants now living in the country unlawfully.

It provides for 20,000 new Border Patrol agents, requires the completion of 700 miles of fencing and requires an array of high-tech devices to be deployed to secure the border with Mexico.

Businesses would be required to check on the legal status of prospective employees. Other provisions would expand the number of visas for highly skilled workers relied upon by the technology industry. A separate program would be established for lower-skilled workers, and farm workers would be admitted under a temporary program.

The basic legislation was drafted by four Democrats and four Republicans who met privately for months to produce a rare bipartisan compromise in a polarized Senate. They fended off unwanted changes in the Senate Judiciary Committee and then were involved in negotiations with Republican Sens. John Hoeven of North Dakota and Bob Corker of Tennessee on a package of tougher border security provisions that swelled support among Republicans.

Outnumbered critics insist the bill falls short of the promises made for it.

Sen. Richard Shelby, R-Ala., called it "the mother of all amnesties."