Though I took notice of Colorado Sen. Cory Gardner’s endorsement of Sen. Marco Rubio for president, I didn’t immediately think about Gardner as a possible running mate for Rubio — until a CQ Roll Call colleague dropped that pearl of wisdom in my lap.

But there are plenty of reasons why Gardner needs to be on any Rubio shortlist of possible running mates, even this early in the 2016 election cycle.

In a party full of elected officials who look and sound angry and bitter, the Colorado Republican invariably is cheerful and optimistic. That doesn’t mean that Gardner is happy with the direction of the country or defends the status quo, but it does mean he is amiable and approachable. Not surprisingly, that makes him appealing to many voters, particularly those who are less ideological and less partisan.