Who will Hillary Clinton choose as her running mate? Here's a hint

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While Ted Cruz may be overly optimistic in choosing Carly Fiorina as his running mate, Hillary Clinton's victories in the Northeast primaries have allowed her to finally look ahead to the November race. Leaks from her advisors show a predictably large group of potential candidates. Clinton can choose anyone, but as past Democratic nominees show, the party's standard bearers have followed a very strict pattern of running mate selection: Look to the Senate.

Since 1940, every Democratic vice presidential nominee except two very notable exceptions has been a sitting U.S. Senator. From Harry Truman to Joe Biden, 13 of the last 15 choices have been taken directly from the Senate. Those two exceptions both stand out -- U.S. Rep. Geraldine Ferraro in 1984 – who was taken from the House -- and Sargent Shriver in 1972. The selection of Shriver deserves a huge asterisk itself. Shriver was George McGovern's second, desperate choice after Senator Thomas Eagleton was picked and then forced to decline the nomination due to revelations about his having received electro-shock therapy treatments. The 1972 and 1984 elections were also noteworthy for a separate reason – those elections represent the two largest Democratic defeats in history.

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Democratic presidential candidate Hillary Clinton

Even before 1940, Democrats followed a very predictable pattern of selection, staying with House, Senate or cabinet members in Washington for their picks. The party has not chosen a sitting governor as a VP candidate since the 103rd ballot fiasco of 1924 -- Nebraska's Charles Bryan was tapped that year.

There is an even longer standing pattern that Democrats have followed -- choosing an official currently holding some elected or appointed office. The last time the Democrats went to the out-of-office bench was in 1908, when the party ran and lost with former Indiana gubernatorial candidate John Kern serving.

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Republicans have historically taken a much different path. Three of the last six Republican VP nominees were out of office when they were nominated, and the only senator they nominated was the widely ridiculed Dan Quayle. However, all but one of those nominees were one-time members of Congress. The only GOP Governor selected was Sarah Palin.

What explains the focus for the Democrats for senators and for both parties on non-governors? Part of the reason may be that senators naturally have a higher profile than governors. Cabinet officials also operate on the national issues, and they bring a connection with the president they served under – which is helpful if that president was popular. Governors may have to answer for a whole grouping of unpopular policies, and also may have to face uncomfortable questions if their state has economic or social trouble during the race. The fact that the Democrats have very few governors after poor performances in 2010 and 2014 present a further hurdle.

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There's another reason that is not particularly applicable to Clinton. As opposed to taking a governor, choosing a senator sends a very pointed message to interest groups. Senators and representatives will almost automatically be called on to vote on hot-button issues, whether it is healthcare, trade, energy or gun control.

These same issues may not really show up in a governor's term. By choosing a running mate who is well known as a proponent on these specific issues, presidential contenders can reassure voters who don't believe the candidate is favorable to their cause. Someone with the long track record of Clinton may not face the same problems. But the fact that the nominee has a track record, and has been vetted by voters in the past, is still usually seen as a positive.

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2016 has already proven to be a year filled with strange new political twists and turns. But a surprise vice presidential selection is not likely to be one of them, at least on the Democratic side of the aisle. So far, the names being mentioned are the usual suspects – senators like Tim Kaine, Sherrod Brown, Amy Klobucher, Elizabeth Warren, Corey Booker, and Mark Warner, cabinet secretaries like Julian Castro or Tom Perez and even some former governors like Massachusetts' Deval Patrick.

While Clinton may break with history in looking outside of the Senate, there is good reason to think she will continue the party focus on the safe choice of an elected or appointed official.

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Commentary by Joshua Spivak, a senior fellow at the Hugh L. Carey Institute for Government Reform at Wagner College in New York. He blogs at The Recall Elections Blog. Follow him on Twitter @recallelections.

For more insight from CNBC contributors, follow @CNBCopinion on Twitter.