This is the CBS Evening News with Nora O'Donnell, reporting from the nation's capital.

Good evening, and thank you so much for joining us. As we come on the air, the Democratic candidates for president are hoping what happens in Vegas tonight won't stay there, but we'll jumpstart their campaigns heading into this weekend's caucus.

It's the ninth debate for Democrats, but there's a newcomer.

Billionaire Michael Bloomberg arrives with a target on his back.

He's facing accusations from his opponents that he is trying to buy the presidency,

that he's made sexist comments as a businessman, and that he promoted racist policies as mayor of New York.

In turn, Bloomberg is expected to make the case he is the only viable alternative to frontrunner Bernie Sanders,

even though Sanders now leads the pack in several new polls.

Today, the Bloomberg and Sanders campaigns went at each other, even sparring over the two candidates' health.

Nicole Killian leads us off tonight from Las Vegas.

How are you feeling today, Senator?

I'm feeling good.

Appearing with picketing union workers in Las Vegas, several Democratic hopefuls made clear

they will demand answers tonight from the newest debater, Mike Bloomberg.

I'm glad he's up there because he's got to answer questions like the rest of us.

We can spend a lot of money. But Bloomberg is sizing up the competition, too.

A new campaign memo warns the fractured field could make Bernie Sanders a Democratic socialist

unstoppable if Biden, Buttigieg, and Klobuchar

remain in the race, it states they will propel Sanders to a seemingly insurmountable delegate lead

by siphoning votes away from Bloomberg. The campaigns today also focused on the heart health

of Sanders and Bloomberg, both 78.

Sanders, who suffered a heart attack last fall, was asked last night if he would release more medical records.

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