A “super PAC” founded by the former TD Ameritrade executive Joe Ricketts is spending more than $600,000 on a television ad in Iowa lashing Senator Bernie Sanders of Vermont as “too liberal” in the final days of his close race against Hillary Clinton in the state’s caucuses.

The spot, being aired by the ESA Fund, describes Mr. Sanders’s policy positions on a range of issues, such as tuition-free college and single-payer health care, saying that it amounts to a flood of new government spending. There will be tax increases on Wall Street, big businesses and the “super-rich,” the ad goes on to say.

The spot is expected to be backed by $600,000 in spending on television ads, and there will be additional expenditures on radio and digital advertising.

Mr. Ricketts has been a major Republican donor this cycle in the presidential race, but with the ad, his group is injecting itself into the Democratic nominating contest.

“When it comes to federal spending and piling on our massive debt, Secretary Clinton is a five-car pile-up, but Senator Sanders is a trainwreck,” said Brian Baker, the president of the ESA Fund. “Given that Senator Sanders is the leading candidate in Iowa and New Hampshire and way ahead in the general election polls, ESAFund will work hard to inform voters about his record and future plans.” Mr. Sanders leads in many head- many national polls.

ESA Fund is the affiliated super PAC of Ending Spending, which was founded by Mr. Ricketts and is operated by his son, Todd Ricketts, who is on the board of the Chicago Cubs. It is laser-trained on fighting what it considers excess spending by the government.

The elder Mr. Ricketts gave $5 million to the super PAC that backed Gov. Scott Walker of Wisconsin, a Republican presidential candidate who dropped out of the race last fall. And Todd Ricketts helped run Mr. Walker’s national fundraising efforts for his campaign.

Michael Briggs, a spokesman for Mr. Sanders, said he had not seen the ad but added, “Are they against strengthening Social Security? Raising the minimum wage? Don’t they want to make college affordable? Do they like Wall Street and big banks? Do they want to keep tax loopholes that let profitable corporations evade taxes?”

Despite the current general election match-ups in public polling, Mr. Sanders is far less known nationally than Hillary Clinton, meaning his negative ratings are lower.

The strategy of attacking Mr. Sanders seems reminiscent of how Senator Claire McCaskill of Missouri, a Democrat, ran ads in her 2012 re-election bid targeting Todd Akin, a conservative and the opponent she most wanted to face because he seemed less electable in a general election. That is also the bar that Mr. Sanders has been fighting to clear.

The Clinton campaign, mindful of moving fast in the final days of the caucuses, quickly seized on news of the Ricketts ad to point out the similarities to the strategy deployed by Ms. McCaskill. In that race, Mr. Akin was used by Democrats as a poster boy for extreme views. Ms. McCaskill, who endorsed Barack Obama in his 2008 presidential primary fight with Mrs. Clinton but who has since become an ardent Clinton supporter, took to Twitter to highlight that point.

“I see you Joe Ricketts. And I know exactly what you’re up to. #ToddAkin Don’t fall for it Iowa Dems,” she wrote.

Outside groups receive less bang for the buck on television ad buys, since they pay much higher rates than campaigns do. It wasn’t immediately clear how many times Iowans will see the spots during a cluttered final week of ads.