Well, Right Wing News has part of the answer.
I also asked my source why he thought so many Republicans had been supporting such an incredibly unpopular bill. He gave three reasons:I think the first reason applies to the Democrats far more than the Republicans. The odds that these new citizens would vote against more freebies from the government seem pretty long. I think the third reason applies only to megalomaniacs like John McCain. The second reason is the key.
First off, there was what he referred to as the "Rovian School of thought," which says that passing this bill would capture the Hispanic vote for the GOP for decades to come.
Next up, there's the "Chamber of Commerce" vote. He says these Republicans were heavily influenced by business groups that want cheap labor no matter what the cost is for the rest of the country.
Then there was the last group, the smallest group in his opinion, who were willing to sign onto a terrible bill just so they could say they were part of a big reform that had bipartisan support.
Businesses that rely on exploiting illegal aliens to make hefty profits are scared to death that the border will be closed. As soon as it happens, they'll have less access to low wage illegals and they'll have to look at hiring Americans and paying normal wages. The bill was being rushed through because the fence is being built. I would bet that if you looked at the legislative calendar and compared it to the construction schedule for the fence, you'd see that this was the last chance to give these business assured access to low wage employees before that labor pool began to dry up. That also explains the Wall Street Journal's editorial page support for the bill.
Don't think of the government as a monolithic entity, either. Since the passage of the fence construction bill last year, there is a part of the government whose jobs and performance reviews depend on construction of the fence. The contractors building that fence rely on the execution of those contracts, too. There is a natural tension between those forces and the amnesty forces going on. The mad rush to get the amnesty bill passed was a manifestation of those tensions.
That's the only reason I can think of to blitz this bill through right now.