Relevant background point #1: Rep. Paul Ryan's fame has depended on his reputation as the man who knew the obscure details of federal budget policy, and who was brave and honest enough to tell the public the unvarnished truth about those details.

Corollary #1: Therefore questions of selectively presented truth, or incomplete honesty, count against his reputation more than they would someone who is seen as a run-of-the-mill partisan. Similarly: suspicions of extra-marital dalliance would do more damage to someone whose image involved being strait-laced than to someone already known as a rogue. (Think: Mitt Romney on the one hand, Bill Clinton on the other.)

Relevant background #2: In his speech at the GOP convention, Paul Ryan really laid on the "selectively presented truths," more than other major speakers from either party. Especially notable:

  • Slamming the Obama administration for Medicare cuts, without mentioning that his budget plan included the same cuts (an omission Bill Clinton smilingly filleted him for in his DNC speech);
  • Slamming Obama for not doing more to support the Simpson-Bowles commission, without mentioning that Ryan was on the commission and voted against its recommendations;
  • Slamming the administration for the downgrade in the U.S. government's credit rating, without mentioning that a big part of the reason* for that downgrade (as reported by Standard & Poor's when issuing the downgrade) was the threat by Ryan and other GOP House members to block a usually routine measure to raise the U.S. debt ceiling and therefore risk a default on U.S. sovereign debt.

Again, we expect politicians to shade and shape their version of reality. But getting a reputation for doing this, as Ryan is doing during the campaign, is a particular problem for someone who has been set up as a uniquely honorable truth-teller.

Which brings us to Ryan and Norah O'Donnell today on Face the Nation. She presented him with another of the partial-truths from his convention speech, which he has repeated afterward. This is his slam of Obama for cutting defense spending, without mentioning that he has voted for these same cuts. I mention this less for what it shows about Ryan than for what it shows about O'Donnell. Take a look for yourself.

Relevant background #3. It is hard for political journalists to know enough about the substance of obscure budget votes to go up against a famed "numbers wonk" from the Budget Committee; harder still for journalists to be sure enough of their knowledge, in the real-time pressure of live TV, to say "no, that's not right" to a national figure; and perhaps hardest of all for a mainstream network correspondent to take on the responsibility of saying, "This does not seem true," rather than just finding some credentialed "critic" to quote to that effect. I've mentioned before some signs of the mainstream media groping to figure out its role in the "post-truth" era. This is an encouraging sign.

On the merits of what O'Donnell was asking Ryan about, see ThinkProgress. This is an issue I have followed. Ryan's thinly shaved rationalization here is consistent with the three I mention from his convention speech, in that each involves the deliberate omission of a major, elephant-in-the-room complicating truth. And, before you ask, when Joe Biden, Barack Obama, or other Democrats go as far as Ryan has -- not in presenting their opinions, or political "visions," but offering gross distortions-through-omission of their own records -- they deserve exactly the same treatment.
* I edited this to include "a big part," since the debt-ceiling uncertainty was a factor but not the only factor S&P mentioned.