As a self-employed contractor, it's quite common for me to negotiate the contract and wait for screening while already working on the project. They still have to pay me for time worked, and if we can't agree, they're screwed as much as I am, because they're already investing in onboarding, and I haven't delivered anything yet.

So there is some room for negotiation, although some things aren't negotiable for them. I've often had a non-compete clause of some sort in my contract, and I've always been able to get its scope reduced, but mever been able to get it dropped entirely. But if you ask up front about it, it's also to their benefit to be honest about it, because hiring you and then having you quit at the last moment because of their lie is not good for them either

The other option is to get the contract signed, or at least seen, before you quit your previous job. If it's a job for which you need to move and dramatically change your life, then you should definitely get some hard commitments from them before you do anything drastic.

In the US employment is at-will, so it's sort of a moot point. Sure they can fire you because of something on your background check, but they could also fire you 3 days after starting because the manager doesn't like you. Short of a serious felony conviction it's unlikely to come up, and if it does discuss it or move on to thousand of other companies.

I faced this off, said I wouldn't sign until after they'd done all their checks.

Then I got radio silence, could see all the checks passed on their end. Didn't end up signing, kept my current job.

Glad I refused. I don't want to work for any company that tries this bullshit.

Don't quit until you sign something that says they will be hiring you and paying you until one of you decides to terminate the agreement?