Washington (CNN) Democratic presidential candidates zeroed in on Michael Bloomberg over the weekend, criticizing his spending and record on issues such as stop and frisk .

The comments reflect growing concern among Democrats as the former New York mayor, who has eschewed early voting states in favor of a strategy that concentrates on Super Tuesday contests, is rising in the polls

"I don't think you should be able to hide behind airwaves and huge ad buys. He has to come on these shows," Sen. Amy Klobuchar told CNN's Dana Bash on "State of the Union" Sunday. "And I also am an advocate for him coming on the debate stage. I know I'm not going to be able to beat him on the airwaves, but I can beat him on the debate stage, because I believe my argument for my candidacy is so much stronger."

"I was not involved in some of the controversial issues in other states like stop and frisk. I understand that that is unconstitutional, but what I was focused on there is trying to go after crimes and making sure there's a consequence, but it does not mean it that always has to be prison time," she said, referencing her time as a prosecutor in Minnesota

Former Vice President Joe Biden , in turn, told NBC that "the point is that $60 billion can buy you a lot of advertising, but it can't erase your record."

"There's a lot to talk about with Michael Bloomberg," Biden said during an interview on "Meet The Press." "You all are going to start focusing on him like you have on me, which I'm not complaining, like you have on me the last six months. You're going to focus on him. His position on issues relating to the African American community, from stop and frisk to the way he talked about Obama."

Asked what type of ally Bloomberg was for the Obama administration, under which Biden served as vice president, he said: "On several issues, like guns, he was a real ally. He was a real ally. But if you notice, he wouldn't even endorse Barack in 2008. He wouldn't endorse him. You know, he endorsed Bush. He endorsed, you know, the Republican before that. All of a sudden he's his best buddy. You know, I mean -- and he would not endorse him."

Biden continued, "You take a look at the stop and frisk proposals. You take a look at his ideas on redlining he's talking about. You take a look at what he's done relative to the African American community."

Bloomberg has repeatedly issued apologies related to the controversial police practice of temporarily detaining, questioning and searching residents who were overwhelmingly black or Latino, including most recently during campaign events in Richmond, Virginia.

"I defended it for too long I think because I didn't understand the unintended pain it caused to young black and brown kids and their families. I should have acted sooner and faster to stop it and for that I have apologized," Bloomberg said during the Virginia Democrats Blue Commonwealth Gala. "I've spent a lot of time with black leaders and community members. I've listened to their stories, I've heard their pain, their confusion, their anger and I've learned from them and I think I've grown from them. I know I can't change history but what I can do is learn from my mistakes and use those lessons to do right by black and brown communities who have suffered."

CNN has reached out to the Bloomberg campaign for comment.

According to a Quinnipiac University poll released last week, Vermont Sen. Bernie Sanders is leading the Democratic primary with 25%, followed by Biden at 17%. Bloomberg is at 15%, up considerably from 9% before the Iowa caucuses and 3% when he first entered the race in November.

"We will not create the energy and excitement we need to defeat Donald Trump if that candidate pursued, advocated for and enacted racist policies like stop and frisk which caused communities of color in his city to live in fear," Sanders said during a Clark County, Nevada, dinner. "The simple truth is that Mayor Bloomberg with all his money will not create the kind of excitement and energy we need to have the voter turnout we must have to defeat Donald Trump."

This story has been updated.