Bernie Sanders, who won Minnesota's presidential caucuses earlier this year, will visit the state next week to campaign for Hillary Clinton, his one-time rival, after recent polling showed her lead in Minnesota narrowing. 

The Vermont senator on Tuesday will attend rallies in Minneapolis and Duluth, two cities where he enjoyed strong support among millennials, a constituency Clinton is now targeting as the general election nears. 

Since endorsing Clinton, Sanders has served as key surrogate against Republican nominee, Donald Trump.Â