The road from the Senate to the White House isn’t easy. Many have tried and many have failed.

Before Barack Obama, John F. Kennedy was the last U.S. senator to take a direct route down Pennsylvania Avenue from Capitol Hill to the White House. And before Obama, no president since Richard Nixon had served in the Senate.

And yet, with the odds stacked against them, the four presidential candidates officially declared so far for 2016 are all senators, or in Hillary Clinton’s case, a former senator.

In the last 25 or so years, Americans shifted from wanting a Washington insider to a Washington outsider in the Oval Office. In 1987, 66 percent said a senator was better prepared to be president to 22 percent who said a state governor was, according to Pew Research Center. In 2014, people were split 44 percent favoring a senator to 44 percent preferring a governor:

Declaring yourself anti-Washington — a popular cry these days — is a hard sell when you’ve chosen to work there. And if you’re a sitting senator like Marco Rubio, Ted Cruz and Rand Paul, there’s voting records, attendance and compliance with congressional ethics rules all available to scrutinize. They’ve already taken hits on all three.

And in the coming weeks and months there will be more votes cast, adding to the treasure trove reporters and opponents mine to highlight inconsistencies or unpopular positions. You have to distance yourself from the dysfunction but still be an engaged public servant.

We asked 2008 GOP presidential candidate Sen. John McCain if he had advice for his three colleagues.

“Remember I’m the loser, alright?” he said, in a brief chat off the Senate floor Monday night.

But to get to the nomination — which he did on a second try — you’ve got to spend a great deal of time letting people “touch you,” particularly in New Hampshire, he said, which of course requires being there for extended periods. And he acknowledged it’s hard to balance bouncing between the Senate and the presidential campaign trail.

“You spend a hell of a lot of time flying back and forth,” he said. “But if you win your constituents are happy. If you lose you have something to answer for.”

(In 2008, our colleague Paul Kane wrote several stories about all the votes McCain missed because he was running for president.)

When Clinton ran the first time she was juggling her full-time job with the campaign, which she doesn’t have to do this time. Monday night, Rubio and Cruz missed a vote on a judicial nominee — to fill a district court seat in Cruz’s home state.

Now Clinton may benefit from focusing all her energy on running for president, but modern history is also not on her side. Clinton would not only join the small club of 20th century senators turned presidents, but, as Pew noted Monday,  she would be the first former cabinet secretary to even be a party’s nominee — let alone win the White House — since Herbert Hoover in 1928.