WASHINGTON — He ran on a promise to end politics as usual.

But on Monday morning, President Trump posted a defense on Twitter of his son’s meeting with a Russian lawyer promising sensitive government information that could be damaging to Hillary Clinton by saying that it was simply politics as usual.

Mr. Trump’s newest tweet about his elder son, Donald Jr., and a controversial meeting on June 9, 2016, at Trump Tower, described the gathering as routine and something almost anyone in his current line of work would do.

“Most politicians would have gone to a meeting like the one Don jr attended in order to get info on an opponent,” Mr. Trump posted on his Twitter account just after 10 a.m. “That’s politics!”

But interviews with politicians by various news outlets have revealed that few elected officials were willing to say they, too, would have taken such a meeting, given how it was described in an email to the younger Mr. Trump.

The president’s defensive stance…