I had written:
Posner is a lawyer: lawyers are used to and see nothing ethically wrong with making strong assertions  in the interest of whoever they take to be their clients about matters of which they are ignorant...
This is going too far. There is no doubt there are unethical and deceitful lawyer. But as your post suggest, there are also unethical and deceitful economist. There are unethical and deceitful people in walks of life.
As a practicing attorney with over 20 years of experience, I can tell you that my colleagues and I hold in low regard unethical and deceitful lawyers. It no more stands for me to accuse academics of being uninterested in the truth, but just career advancement or identity politics or academic politics or what have you than it stands for you to accuse the legal profession of disregarding the truth. The case of the criminal John Yoo, who is both a lawyer and a member of the faculty of UC Berkley, should give us both pause.
Lest you think its o.k. for lawyers play fast and lose with the truth, rule 4.1 of Model Rules of Professional Conduct provides:
In the course of representing a client a lawyer shall not knowingly:
(a) make a false statement of material fact or law to a third person; or
(b) fail to disclose a material fact to a third person when disclosure is necessary to avoid assisting a criminal or fraudulent act by a client, unless disclosure is prohibited by [the rule governing client confidences]. http://www.abanet.org/cpr/mrpc/rule_4_1.html
Although the rule does give lawyers some leeway in not disclosing untruths, it categorically proscribes the making of false statements. Interestingly, there is no comparable provision regarding truth telling in the Federal Code of Judicial Conduct, the code governing Judge Posner’s conduct, that I could find. However, commentary to that code provides, “A judge should avoid lending the prestige of judicial office to advance the private interests of the judge or others.” My guess is that Posner, if pressed, would take the view that his carrying water for the Republicans is not done to advance a private interest.
I am not naïve. The sad truth is that many lawyers and others view lawyers as mere hired guns. Remarkably, I have encountered acceptance of this role for lawyers more frequently among professors at elite law schools than among fellow practioners.
Look: Richard Posner has no idea whether any of the $40 billion in stimulus tax cuts that the Office of the Treasury's Fiscal Assistant Secretary reported to Christy Romer had been spent were calculated on an accrual rather than a cashflow basis. He has not lifted a finger to try to find out. Yet he is happy to assume that all are accrued liabilities rather than changes in cash flows and to carry out his analysis as if that assumption is a fact.
Richard Posner calls this zealous advocacy for someone--the Republican Party--whom he regards as the equivalent of a client.
I call this misleading.
I leave it to you whether this is a violation of rule 4.1 of Model Rules of Professional Conduct. (I don't think it is.)
Are you suggesting that Posner does have evidence that the $40 billion in stimulus tax cuts that the Office of the Treasury's Fiscal Assistant Secretary reported to Christy Romer had been spent were calculated on an accrual rather than a cashflow basis?
 UPDATE: And, indeed, U.S. Treasury staff report that Richard Posner's claim that the $40 billion number they gave Christy Romer is some sort of accrual number is simply false. It is a cash flow number.