Mitt Romney's path to the White House is getting ever narrower as a new round of polls shows him lagging in Iowa, Colorado and Wisconsin—three states where he had hoped to find new stepping stones to an Electoral College majority.

In Iowa, President Barack Obama has opened a 50% to 42% lead among likely voters in a Wall Street Journal/NBC/Marist poll, just four months after Marist found the two candidates in a dead heat.

Mr....