I understand that Roth recharacterizations have been eliminated for
Not true. So basically, before 2018, there were 3 types of "recharacterizations":
- Recharacterize a Traditional IRA contribution to a Roth IRA contribution
- Recharacterize a Roth IRA contribution to a Traditional IRA contribution
- "Undo" a conversion from Traditional IRA to Roth IRA (or from Traditional 401k to Roth IRA) and leave it in Traditional IRA
Only #3 has been eliminated for conversions that were done in 2018 or later. #1 and #2 are unaffected.
So you can definitely still recharacterize (all or part of) your Traditional IRA contribution into a Roth IRA contribution before April 15, 2019 (or October 15 if you requested an extension or if you filed your taxes on time before April 15). What this means is that it will count for tax purposes "as if" you had made it originally as a Roth IRA contribution in the first place, at the original time that you made your Traditional IRA contribution. This means that you must have been eligible to make that much of a direct Roth IRA contribution for 2018 (e.g. you must be below the income limits). Since it's treated as having been in Roth IRA all along, and not converted from Traditional IRA to Roth IRA, there is no tax on conversion.
Am I allowed to simply convert the non-deductible portion of my 2018
Traditional IRA contribution to a Roth IRA
You can "convert" an amount of your Traditional IRA into Roth IRA. You cannot choose for the conversion to only come from after-tax funds. As Ben Voigt commented, there is a "pro-rata rule" for distributions and conversions from Traditional IRA which means that the distribution or conversion will be considered to consist of a mix of pre-tax and after-tax funds in the same proportion as in your Traditional IRAs overall. It doesn't make a difference if you keep the money in different Traditional IRA accounts -- all accounts are considered together for the pro-rata rule.
If you had a significant amount of pre-tax funds in Traditional IRA from before, the conversion will likely consist of a significant fraction of pre-tax funds, and conversion of pre-tax funds is taxed (and it will be taxed in the year you make the conversion, i.e. 2019); and the remaining funds in your Traditional IRA will also consist of a mix of pre-tax and after-tax funds. On the other hand, if you did not have any money in Traditional IRA from before, and you will not contribute or rollover any pre-tax money into Traditional IRA for the rest of the year, then the pro-rata rule will only count the after-tax contribution and the pre-tax earnings from when you contributed till now. In that case, if you converted them entire balance of your Traditional IRA, on conversion you would only pay tax on the (likely small) amount of earnings.