No, an Employer should not change your Federal Withholdings without your consent unless they receive a letter from the IRS stating they must do so. In this case, the IRS would also send you a copy of the letter.
Other than an order from the IRS, your Employer should not change your Federal Withholdings except when you submit a new Form W-4.
Keep in mind, tax rates for 2018 will be lower, so your Federal Withholdings will be lower as well. The IRS recently changed the Federal Withholding tables to reflect the change in tax rates.
