After months of work by backers in both parties, the Senate is ready to take the first procedural votes Tuesday afternoon on sweeping immigration reform legislation, starting an intense debate on the future of millions of who illegally live in the United States.

"The time is now for the Senate to act," said Sen. Patrick Leahy (D-VT), who will try to shepherd this bipartisan bill to approval in coming weeks.

While Leahy expressed optimism that the full Senate will approve this bipartisan bill, Democrats will need the votes of maybe as many as 10 Republicans to garner the 60 votes needed to advance this bill.

Much of the pressure to find those votes will fall on the shoulders of Sen. Marco Rubio (R-FL), who has tried to sell Republicans on the bill for months; Rubio argues at this point, the plan still might need some tweaks to toughen border security measures.

"If we cannot secure the border, if we cannot take the necessary steps to earn our colleagues’ trust, this will never become law," Rubio said in an interview earlier this week on Univision.

"A year ago, to say that Republicans would have been full partners in an immigration reform bill, I mean, people would have laughed at that," said Tamara Jacoby, who leads ImmigrationWorks USA.

Jacoby told an audience in Washington, D.C. on Monday that she still isn't sure the deal struck by four Senators in each party will hold, as the balance between more border security and a path to citizenship for those in the U.S. illegally is a difficult needle to thread.

"This is certainly going to be one of the more contentious issues," Jacoby said.

And that was obvious on the Senate floor on Monday.

"The bill does not secure the border," said Sen. Mike Lee (R-UT), who has strongly criticized the Gang of Eight product.

"It will, however, immediately legalize millions of currently undocumented immigrants, make them eligible for government services and put them on a pathway to citizenship," said the Utah Republican.

The first vote on Tuesday wlil be to shut off debate on the motion to proceed to the bill; that will get more than 60 votes - but when it does - don't assume that it means the bill will pass.

We don't have to look back very far in the U.S. Senate to find an example of that, as just earlier this year, the Senate easily voted to start debate on a major gun control bill.

A number of GOP Senators voted to start debate; not long after that, many of them then refused to join in a compromise plan on background checks for gun purchases.

Yes, the debate will officially begin on Tuesday afternoon on immigration reform. What comes next remains unclear.

As for the White House, they are clearly gearing up for a big push, as President Obama set a Tuesday morning event, with this sent to reporters on the event:

After months of work by backers in both parties, the Senate is ready to take the first procedural votes Tuesday afternoon on sweeping immigration reform legislation, starting an intense debate on the future of millions of who illegally live in the United States. "The time is now for the Senate ...