Harry Reid just announced he’ll hold a key cloture vote tonight on the Corker-Hoeven amendment, which would dramatically beef up the Senate immigration bill’s border security provisions. It is expected to pass by a comfortable margin, which in turn all but ensures that the Senate bill will pass this week — a major step forward for reform.

One of the leaders of the conservative drive to derail reform is Senator John Cornyn, who had proposed his own amendment for hard security triggers as preconditions for citizenship — an apparent effort to scuttle reform that was beaten back by the gang of eight and replaced with Corker-Hoeven. Today Cornyn announced his opposition to that replacement amendment, and his statement underscores the degree to which conservatives are running out of arguments against the Senate bill:

I cannot support an amendment cobbled together at the eleventh hour that doubles the border patrol without knowing how much it will cost or whether it is even the right strategy.

Yes, Cornyn, who himself proposed adding 5,000 border patrol agents, now opposes Corker-Hoeven because of the potential cost of that proposal’s additional agents…even though the nonpartisan Congressional Budget Office finds the Senate bill will substantially reduce the deficit.

This comes after Senator Ted Cruz argued that Republicans should oppose the immigration reform bill in the name of undocumented immigrants, and after Senator Jeff Sessions argued that Republicans should oppose the immigration reform bill in spite of its projected economic benefits, because they will supposedly go only to business owners. Meanwhile, Sessions and around a dozen other opponents just fired off a letter to Harry Reid demanding still more time to debate the bill. Of course, Sessions has openly made it known that the call for procedural delays is all about stalling long enough to build up public opposition in hopes of killing it. As he puts it: “The longer it lays in the sun, the more it smells, as they say about the mackerel.”

Is anyone else beginning to get the sense that all of these arguments and procedural objections aren’t really grounded in substance?

By the way, here’s something else to keep an eye on: how will Chris Christie’s replacement in the Senate, Republican Jeff Chiesa, vote on Corker-Hoeven and on the final bill? He has repeatedly refused to say, and that is spreading consternation on the pro-reform side, because every vote in the Senate counts, since the higher the vote total, the more pressure there may be on the House to act. A spokesman for Chiesa confirms to me he still hasn’t taken a position, with time running out.

If Chiesa does vote against Corker-Hoeven or against the bill, reform-minded Republicans will likely cast doubt on Christie’s pretensions to being a pragmatic and non-ideological Republican himself. “If Christie is the moderate problem-solver with independent appeal he portrays himself to be in the media, he’d strongly support comprehensive immigration reform,” one aide to a reform-backing GOP Senator tells me. “There’s no expectation that Christie would ever tell the senator he appointed how to vote, but we hope he offers some private advice and encourages a vote for reform.”

That said, it seems very likely that Christie’s replacement will, in the end, vote Yes on both Corker-Hoeven and the Senate bill. Which would again underscore the deepening conservative isolation on immigration we’re now seeing.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.