Menedez would not back Obama on some key issues and publicly came out against some of Obama's policies. Normally these "conspiracy" allegations would be ignored at the DoJ and Obama would claim prosecutorial discretion or simply flat out denial and claim the "vast rightwing conspiracy" theory. However, this is what happens when a Democrat won't play ball with their leadership. A minor financial issue now gets the spotlight and is, I believe, primarily politically based. Menendez will be the "example" the DNC will use with their members as he's hung out to dry by his own. Some Republicans will certainly jump on the bandwagon because well... that's what partisan's do. The media certainly wasn't pushing for Menendez's prosecution - no one was digging to oust him other than some rightwing internet sites when the story first broke. Otherwise, this was an absolute snoozer of a scandal. Hell, even here in NJ it was page 4 below the line stuff.
Let's remember this is the same DoJ who refused to prosecute the IRS, the VA, various voter fraud / intimidation,
improper campaign donors. Obama and the DNC need a whipping boy and Menedez fits the bill.