The Fix's Aaron Blake breaks down why Democratic presidential candidate Hillary Clinton chose Sen. Tim Kaine (D-Va.) to be her running mate. (Whitney Leaming/The Washington Post)

Hillary Clinton isn't someone who takes many risks.  She's a meticulous thinker who almost never leaps before looking. That caution has not always worked in her favor, politically speaking, but it is, without question, her defining trait.

Clinton's selection of Tim Kaine, the Virginia senator, to be her vice-presidential running mate reflects that caution -- coupled with a confidence that this general election race is hers to lose.

From the start of the vice-presidential selection process, Kaine has always been at or near the top of any list of potential contenders. The reason is simple: He checks lots and lots of the traditional boxes one looks for in a vice president.

Kaine comes from a swing state. He has executive and legislative experience. (Before going to the Senate, Kaine was governor of Virginia from 2005 to 2009 and mayor of Richmond prior to that.) He has a strong Catholic religious background. (He was a missionary out of college.) He speaks fluent Spanish. He had been vetted favorably by then-Sen. Obama during the 2008 presidential campaign.

And, most importantly, Kaine is a steady presence. He has been in the national spotlight -- both during the 2008 vetting and during his stint as Democratic National Committee chairman from 2009 to 2011. He knows how to handle the media, the scrutiny and the attacks that come with a high-profile perch. He's even-keeled amid chaos.

The flip side of all of that is that Kaine is occasionally -- okay, often -- described as "boring." "I am boring," he joked in an interview with "Meet the Press" host Chuck Todd last month. But the fact that Kaine is more workhorse than showhorse is what commended him to Clinton.  Far from being a negative, Kaine's steadiness -- call it boringness if you want -- was a huge positive.

Remember that running mates tend to either reinforce or undermine the broader narrative the presidential candidate is trying to sell. When Bill Clinton picked Al Gore in 1992, it was a doubling down on the fresh-faced sons of the New South. (That was good.)  When John McCain named Sarah Palin in 2008, it undermined his key message against Barack Obama of experience vs naivete. (That was bad.)

In naming Kaine, Clinton is making it clear that she favors policy chops over pizzazz, governing over glitz.  She hopes that drives a very stark contrast with the showmanship and celebrity of Donald Trump.  Trump is talk, Clinton is action.

Clinton/Kaine isn't the world's most exciting ticket. That's on purpose.  Clinton clearly believes that the electoral map and the demographic realities of the country favor her. And that as long as she does nothing to roil the waters, she is likely to win in November.

If Clinton felt as though she needed to either court the liberal left or more broadly shake up the race, she would have chosen someone like Sen. Cory Booker, an African American, or Elizabeth Warren, a liberal icon.  But Clinton didn't, and, in truth, I'm not sure how close she ever came to picking anyone other than Kaine.

The Kaine pick is Clinton sending a very clear message: This is my race to win, and I'm not going to take any unnecessary risks along the way that could potentially screw that up.