You refer to an audit (perhaps politcally motivated) of one of his income tax returns. The statute of limitations had expired on that year (meaning the IRS could not legally assess any additional tax), but Nixon voluntarily waived the statute and paid the additional tax due. You'll never see any democrat do that. The issue of whether Nixon had any crimnal evasion possibilities was never proven, otherwise the rats would have used it immediately.
I would be surprised to hear that the Judiciary Committee had recommended to the House an article of impeachment regarding a tax return that could not be proved.
Also, are you sure about the statute of limitations? There may be a time limit to challenging the accuracy of a tax return for purposes of amending the tax due. This is not necessarily the same as a statute of limitation concerning commission of a felony by use of a forged document. Perhaps you can clarify what you are suggesting and also suggest a reason that Nixon paid the tax.