UPDATE 4:11 p.m.: Historic immigration legislation cleared a key Senate hurdle Monday afternoon with a 67-to-27 procedural vote in favor of the "border amendment" seen as key to gaining Republican support for the bill.
The final tally was seven more than the 60 needed, with 15 Republicans voting to advance the legislation.
PREVIOUSLY: The U.S. Senate is poised to vote on a tough border security amendment to the immigration bill Monday evening. The measure is designed to entice Republicans to vote in favor of the larger immigration reform proposal. Business and labor groups are also putting pressure on Senators ahead of that vote, which is expected on Thursday.
Momentum is growing for passage of the comprehensive bill.
Republican Lindsey Graham of South Carolina – one of the "Gang of Eight" who negotiated immigration reform – says the measure is "on the verge" of getting 70 votes, thanks to the enhanced security amendment.
The "surge" amendment would spend $30 billion to add more border patrol agents. Senator John Hoeven of North Dakota, who co-authored the amendment, says it would provide enough funding to potentially station an agent every 1,000 feet and to complete the fence along the U.S.-Mexico border.
New pressure is coming from the U.S. Chamber of Commerce, which came up with a compromise with the United Farm Workers union on a temporary guest worker program included in the bill. The chamber is running radio and TV ads aimed at securing those Republican votes. It features GOP "Gang of Eight" member, Marco Rubio saying: "we all wish we didn't have this problem. But leaving it the way it is, it's amnesty."
The ad ends with a female voice urging viewers to call Congress to "end defacto amnesty" and support "conservative immigration reforms."
The United Farm Workers union is using social media to get its message out. Farm workers in the fields will be using their lunch break to post “Photos from the Fields” on their Twitter and Facebook accounts, asking lawmakers to support the Senate immigration bill. They'll be using the #FieldFotos hashtag.
There is still strong opposition. More than a dozen GOP Senators wrote to Majority Leader Harry Reid today complaining that few of the more than 300 amendments to the bill will be given a vote. “After repeated promises of a full and open amendment process on the floor," they wrote, "we have had nine roll call votes." They called it "deeply disturbing."
Earlier on Monday, business leaders met with President Obama at the White House to talk about immigration reform. The President touted the contribution of immigration to job creation and described the bipartisan Senate bill "strong" even though "it's not a bill that represents everything that I would like to see." He urged the Senate to pass it "so that we can then move to the House and get this done before the summer break."
White House spokesman Jay Carney told reporters Monday that's "going to be a heavy lift."
The GOP-led House of Representatives is working on a series of immigration bills dealing with workplace enforcement and high tech visas. The bipartisan group working on a comprehensive measure – that includes a pathway to citizenship – has not yet produced a bill.