Tuesday's special elections for House seats in New York and Nevada were devastating for Democrats. Both races turned into a referendum on President Obama, who again proved how unpopular he is.

The New York seat made vacant by Anthony Weiner's sexting scandal has been in Democratic hands since 1923. Mr. Obama carried it by 11 points in 2008, but on Tuesday Republican Bob Turner beat Democrat David Weprin by eight. In a district with a 3-1 Democratic registration advantage, dissatisfaction with Mr. Obama among blue-collar ethnics and orthodox Jews was central to the GOP victory.

John McCain carried Nevada's 2nd District by a mere 88 votes in 2008. Yet Tuesday, former state Republican chairman Mark Amodei beat State Treasurer Kate Marshall by 28,307. (Full disclosure: I'm associated with a group—American Crossroads—that spent $265,000 on mail, phones and online media to drive up GOP early voting in the race.)

In some ways, the Nevada defeat is the more damaging. While both Democrats were lavishly funded, Mr. Weprin is now being disparaged as a poor campaigner. However, Ms. Marshall was thought to be the perfect candidate. Running the 2012 Democratic playbook, she ran ads accusing Mr. Amodei of "supporting an end to Medicare to give tax breaks to millionaires." It didn't work.

The Obama presidency is now in a downward spiral. The economy has flat-lined, his poll numbers are plummeting, and Democratic leaders are turning on the president.

Karl Rove is a former senior adviser and deputy chief of staff to President George W. Bush. He is a Fox News contributor and author of "Courage and Consequence" (Threshold Editions, 2010). To continue reading his column in The Wall Street Journal, click here.