NEWARK - Sen. Cory Booker says he believes New Jersey Gov. Chris Christie is the most underestimated presidential candidate in the crowded 2016 field.

Booker spoke Monday in Newark at an event held by the news organization Politico.

The Democrat and Christie have long been friends despite their different political parties. Christie's campaign has been struggling to gain momentum.

Booker also bemoaned the state of the region's transportation infrastructure, which he says is in crisis.

New Jersey Transit and Amtrak trains suffered major delays last week that infuriated commuters. Booker says the nation should be ashamed of itself for letting its infrastructure deteriorate so badly.

The former Newark mayor has also been talked about as a potential vice presidential candidate. But he says that position's not a focus of his right now.