My personal advice? Don't even go there! (Needless to say, as a lawyer, I have to explain that this is only personal advice, not legal advice.

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The problem is that the determination of whether these people are governmental employees is governed not by the terms of the contracts with the leasing agency, but by the traditional 20-factor test as interpreted by cases such as Microsoft. Since these tests are as clear as mud, you may well discover that the employees argue that they are employees of either the government agency or the leasing company, whichever happens to be the most favorable to them at any particular point in time. And since the issue is determined on an employee-by-employee basis, even if you are successful in litigation with one employee, the next one can still come along and argue the same issues.
Some examples: suppose you decide that as leased employees, these individuals are not covered by a state retirement system which would have covered them had they been employees of the entity which leases them. Given that the state statute does not contemplate leased employees, the statute presumably includes some general language to the effect that "employees" in certain categories are covered. A leased employee could argue that even though the contract with the leasing agency says s/he is employed by the leasing agency rather than by any governmental entity, s/he is actually a common-law employee of the governmental entity, and is therefore covered under the state retirement system.
And of course, the employee cannot be expected to argue this during employment, when employee contributions would be due. Instead, the argument would typically come up at retirement, when the employee would argue entitlement to a pension. That means that by the time the issue would even be litigated, you would have a lot of affected employees, and the potential for a class action suit against you, as well as not having collected employee contributions for all those years. And if the leased employee were also covered under a plan of the leasing agency, s/he might even end up with double coverage.
Conversely, suppose that the state retirement system
does cover these individuals. In that instance, a leased employee could argue that s/he was not a governmental employee, and therefore that the plan was, at least as to that employee, a plan covered by ERISA. This argument would be particularly likely if the employee died at a point when s/he was not entitled to benefits. A surviving spouse could argue that the preretirement survivor annuity provisions of ERISA applied, and therefore that the spouse was entitled to a retirement benefit which the retirement system never contemplated.
Or suppose that the leased employees are covered by a plan maintained by the leasing agency. As an entity covered by ERISA, the leasing agency would normally have the right to cease benefit accruals at any time. However, an employee could argue that because s/he was really an employee of a governmental entity, federal and/or state Constitutional principles would preclude any diminution of the future benefit formula as applied to employees hired before the date of any such change.
The best idea here would be to pay the leasing agency to perform administrative services, if needed, but to treat the individuals for all purposes as employees of the governmental entity. If you want to cut them out of particular benefits, amend the plan document to exclude them, rather than trying to argue that they are someone else's employees. Of course, this doesn't work in the case of a 403(B) plan which includes salary reduction contributions. And if you've got a state plan covering local employees, this may require some cooperation from your state legislature.
And yes, I do understand the political pressures in many states to "privatize" employees, even if in practice they stay at the same desks performing identical jobs. In some instances, we have been able to come up with some fuzzy language which causes people to be arguably "privatized" while still being treated as governmental employees for withholding and employee benefits purposes. But trying to treat employees as leased from an agency can bring you into litigation which can be expensive even if you win, and can be lots worse if you lose.
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