Missouri’s Claire McCaskill Tuesday ostensibly became the first U.S. Senator to formally back Hillary Clinton for president in 2016.

“I’m proud to announce that I am Ready for Hillary.  And — as I said back in February — if Hillary decides to run, I will work my heart out to see that she is elected,” McCaskill said in a statement put out by the pro-Hillary group going by the ‘Ready for Hillary’ moniker.

In fact, McCaskill actually already backed Hillary when speaking to reporters in Missouri four months ago – but national press, which is fond of repackaging old news, will stubbornly mark June as the official date.

Regardless, McCaskill’s incredibly early endorsement is a ripe example of the power the Clintons hold over friends and rivals alike.

McCaskill  irked the Clintons in January of 2008 when she became one of the first female elected officials to cross the Rubicon and endorse Barack Obama after the New Hampshire primary.

But McCaskill’s relationship with Bill and Hillary was strained dating back to 2006 during her first U.S. Senate run.

When asked by NBC’s Tim Russert during a Senate debate whether she thought Bill Clinton was a great president, McCaskill replied, “I think he’s been a great leader but I don’t want my daughter near him.”

Well-sourced Democrats say the Clintons viewed that particular comment beyond the pale, potentially permanently damaging the relationship.  McCaskill, an astute pol herself, certainly knows this.  So it’s in her interest to be out early and often for Hillary — if she hopes to have any type of sway under a President Clinton II.

The Clintons don’t forget, which is why the pressure on Democrats to get aboard Hillary’s train early — or at least keep quite — will be enormous.