The conservative group Club for Growth is hitting the airwaves in Iowa, less than a week until the caucuses, with an ad campaign attacking Democratic presidential contender Sen. Bernard Sanders as “too liberal” and “more radical than Obama.”

In keeping with the political tradition of meddling in opposition primaries, the free-market advocacy group acknowledged that its ad campaign could boost Mr. Sanders. Officials with the group said they don’t want to see him get “robbed” of the Democratic nomination again and that they look forward to the primary contest extending to the Democratic National Convention in Milwaukee in July.

“Bernie Sanders wants to be our 46th president — but he’s nothing like the rest,” says a narrator in the 30-second ad, saying he’s “more radical than Obama on health care” and criticizing his “socialist Green New Deal.”

The ad is backed by a $41,500 buy and is scheduled to run in the Des Moines area on news and political shows through Sunday, the group said.

“With Bernie Sanders rising in the polls, the potential for a radical socialist as the Democratic nominee has never been more real,” said David McIntosh, the president of Club for Growth Action.

The group also acknowledged that it could serve to give Mr. Sanders a boost by solidifying his grassroots base with a targeted ad from the right.

It’s not clear how effective the effort will be.

Some groups have tried similar schemes in the past by running “attack” ads in the opposing party’s primary characterizing a candidate as too “extreme.”

The idea is that the ads will motivate the far-right or far-left voters most likely to participate in a lower-turnout primary to support that candidate, who might be perceived as a better match-up in a general election.

In 2012, former Sen. Claire McCaskill’s campaign ran ads ahead of a Republican primary contest attacking former Rep. Todd Akin as “the most conservative congressman in Missouri.”

Mr. Akin won the primary but lost to Ms. McCaskill in the general election after his comments about “legitimate rape” helped sink his campaign.

“I speak with authority … this ad is meant to help Bernie. Not hurt him,” Ms. McCaskill tweeted.

Conservative radio host Hugh Hewitt, a prominent defender of President Trump, announced recently that he plans to vote for Mr. Sanders in the March 3 primary in Virginia, where voters don’t register by political party.

Virginia Republicans opted to skip a presidential primary this year and pick delegates to the Republican National Convention at a party convention.

Laura Hubka, the Howard County, Iowa, Democratic Party chairwoman, voiced skepticism that outside efforts to elevate a “weaker” opponent would pay off.

“They talk about it every election,” Ms. Hubka said. “I don’t see hundreds of people doing that. That would be a big, combined effort on someone’s part.”

Participants in the Iowa caucuses can register or change their party registration on-site Monday.

In New Hampshire, site of the first-in-the-nation primary on Feb. 11, “undeclared” voters can participate in either party’s primary.

Deb Nelson, chairwoman of the Hanover/Lyme Town Democrats in New Hampshire, suggested that people should be careful what they wish for if they’re looking to throw a wrench in the other side’s plans.

“Four years ago, I spoke to no small number of people who were registered independent, tend to vote Democratic and said they were going to vote in the Republican primary because they wanted to vote for Donald Trump and mess up the Republicans,” she said. “Which turned out to be just a horror story.”

Other results haven’t been as clean as Ms. McCaskill’s situation, either.

In 2018, Rep. Sharice Davids, Kansas Democrat, accused former Rep. Kevin Yoder, a Republican, of meddling in the Democratic primary after a GOP-aligned outside group labeled Davids’ Democratic opponent Brent Welder an “Obama-Sanders” progressive and “too progressive for Kansas.”

Ms. Davids went on to win the Democratic nomination and defeat Mr. Yoder in the general election.

In 2008, conservative radio host Rush Limbaugh started an effort he dubbed “Operation Chaos” in which he urged listeners to back Hillary Clinton over Barack Obama, who was leading in the Democratic presidential primary contest at the time, under the logic that a drawn-out primary would damage the Democrats’ chances in the general election.

Former Republican Rep. Barbara Comstock of Virginia has acknowledged voting for Mr. Obama in the 2008 primary, figuring he would be easier to defeat in a general election than the well-oiled Clinton political machine.

But Mr. Obama won the nomination over Mrs. Clinton and defeated GOP nominee John McCain in November by a relatively comfortable margin.

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