Washington (CNN) On Tuesday, something interesting -- and important -- happened in the battle for the Senate.

Senate Majority PAC, the chief super PAC of Senate Democrats, dumped $3 million worth of TV ads into the New Jersey Senate race , where Democratic Sen. Bob Menendez is facing a surprisingly strong challenge from wealthy Republican businessman Bob Hugin.

The first ad from the super PAC seeks to link Hugin with President Donald Trump, who is not particularly popular in the Garden State.

That the Senate Democratic establishment -- and make no mistake, that's what Senate Majority PAC is -- is spending money there is evidence of two things:

1. Menendez is being drastically outspent on TV by Hugin, who has dumped $24 million of his own money into the race, and Democrats are feeling pressure to level the playing field. (Another pressure point: Menendez is a former chairman of the Senate Democratic campaign committee, meaning that if he needs national donor money spent to save him, he gets national donor money spent to save him.)

2. Democrats are worried.

Most polling suggests Menendez is comfortably ahead. (The Real Clear Politics average lead for Menendez is 7 points.) But Menendez has lingering negatives from the trial he endured over accusations of corruption . (The 11-week trial ended in a hung jury and the Department of Justice declined to retry him.) And New Jersey -- because it doesn't have its own media market and is instead covered by Philadelphia and New York City -- is a difficult state for politicians to become known in.

All those facts, of course, don't change this underlying reality: Senate Democrats are spending $3 million in New Jersey that they can't spend in, say, Tennessee, where former Gov. Phil Bredesen, a Democrat, is trying to swim upstream against the conservative nature of the state. Or in Arizona, where Democratic Rep. Kyrsten Sinema is trying to win retiring Republican Sen. Jeff Flake's seat.