Bill Clinton's popularity transferrable to Barack Obama?  So Geraldo Rivera wondered on Fox & Friends on Thursday morning.  As must most Democrats, who have growing concerns about President Obama's reelection chances.   

But if endorsements by popular past or sitting presidents matter so much to the outcome of elections, why did Dick Nixon lose to Jack Kennedy in 1960?  Eisenhower -- whom Nixon served as vice president - could have easily won a third term in office had he not been barred constitutionally.  Nixon lost a close contest to Kennedy full of voting irregularities, but Eisenhower's glow should have made it a cakewalk for Nixon.   

And what the heck happened in 1992?  The late Ronald Reagan stumped for his successor, George H. Bush.  Mr. Reagan, whose integrity and popularity far eclipsed Mr. Clinton's, did nothing to stop voters from ousting Mr. Bush.

Further back, in the 1938 midterm elections, FDR couldn't prevent voters from tossing out dozens of congressional Democrats.  

Plus, Bill Clinton has a decidedly mixed record in the endorsement department, as Roll Call explained on Wednesday.  In 2010, a heavily campaigning Mr. Clinton failed to stop the GOP from capturing the U.S. House.  Democrats lost the House by margins not seen since the 1940s. 

Endorsements are generally overrated, whether by illustrious politicians or celebrities.  What Mr. Clinton could do for Democrat candidates is raise their profile and raise money.  Beyond that, voters strongly tend to focus on the merits or demerits of a candidate and vote accordingly (at least those who aren't straight ticket voters). 

What Mr. Clinton's lengthy speech endorsing Mr. Obama does for the president by Election Day won't matter much.  Voters aren't touching screens for Mitt Romney or Bill Clinton on November 6.

For Barack Obama and down-ballot Democrats that could be a big problem.