Let’s address the questions of the day right off the bat:
Are Scott Walker’s sweeping tax cuts – property, income, withholding tables – in danger?
Answer:  Not really.
Is Senator Mike Ellis blocking them?
Answer: No.
RightWisconsin’s Brian Fraley addressed the same questions earlier today and came up with different answers.  Fraley wrote that Ellis is "leading a small but vocal group of colleagues who do not favor the Governor's tax cut plans" and that Ellis and the others "are standing in the way of the sweeping tax cuts."
This is not what my sources are telling me. I spoke with half a dozen members of the GOP caucus as well as Walker administration officials. 
No one I spoke with said that Ellis was "standing in the way" of the tax cut. 
To the contrary, the governor’s office says that Ellis has been working with them on mutually agreeable "tweaks." 
And, late this afternoon, I spoke with Ellis himself, who says he is on board with the tax cut package.
"We're on board. We're ready to go," he said. 
Ellis says that he and Walker’s office agreed on a technical change that will have the effect of reducing the projected  $807 million structural deficit.  This is wonky, but it works this way:  the plan would shift the $117.4 million that had been earmarked for the rainy fund  to the state’s  current statutory balance, which is now  $65 million. That new balance could then be carried over to the next budget – reducing the projected deficit in that budget. 
Yes, it looks like a bit of accounting sleight of hand, but it means that GOP legislators will be able to boast they had cut the "structural deficit," and also clears the way for Walker’s full property and income tax cut.
My sources now say that there are 16 solid GOP votes for the tax cut plan, just one short of the 17 votes needed for it to clear the senate. The lone hold-out appears to be Senator Rob Cowles, a longtime ally of Ellis's. Like Ellis, Cowles has long worried about structural deficits and he continues to express concern. In fairness, those concerns are substantive and consistent with his fiscal record. They are also likely to be addressed in the next few days.
So where does that leave us? The broad consensus of everyone I spoke to was that, in the end, there is only a slim chance the tax cuts would fail. 
Speaker Vos's Assembly is firmly united behind the cuts, and that will put enormous pressure on their colleagues in the upper house. The jockeying for precedence between the two houses may lead to complications, but these are likely to be speed bumps, not landmines.  The Assembly will pass the tax cuts next week, the senate will then refer them to Joint Finance, which is likely to tweak the bill with minor fixes… and it will then go to the full senate.
With Ellis on board, observers think it is unlikely that Cowles would play the role of last-minute spoiler on an issue so important to Governor's Walker's agenda.