DES MOINES, Iowa >> The cavalry arrived too late for Gov. Scott Walker, but other candidates struggling in the shadow of Donald J. Trump hope their own backup — in the form of super PACs flush with money for television commercials — will soon be riding to their rescue.

Even as Walker pulled the plug on his presidential campaign this week, a super PAC supporting him had just begun spending lavishly on TV ads in hopes that saturating the airwaves would keep him viable.

It never got to test that premise. But other super PACs, which can accept unlimited sums from people and corporations, are forging ahead, committing multimillion-dollar budgets to thousands of 30-second ads in a coming air war over Iowa, New Hampshire and South Carolina.

The sums reflect the high stakes as lagging candidates fight to connect with primary voters who have so far rejected them in favor of nonpoliticians like Trump, business executive Carly Fiorina and Ben Carson, a retired surgeon.

A super PAC backing Jeb Bush will spend $37 million on ads through February, money it is forced to spend early because Bush’s popularity with elite donors has not been matched by support from voters. Rival super PACs and allied groups are also set to run TV ads primarily in early voting states for Gov. Chris Christie of New Jersey ($11.4 million), Sen. Marco Rubio of Florida ($15 million) and Gov. Bobby Jindal of Louisiana ($5 million), according to a firm that tracks political ad spending.

None of those groups plan to attack Trump any time soon, according to their strategists, for fear of repercussions.

"When you go negative on advertising, the natural physics of doing that increases your unfavorables," said Matt David, a strategist for the super PAC supporting Gov. John R. Kasich of Ohio.

David’s group, New Day for America, saturated New Hampshire with more than 1,500 spots as Kasich surged past Bush in New Hampshire polls in late August. After the second Republican debate last week, the group booked three more weeks of ads at a cost of $750,000, including one that subtly mocks Trump’s helicopter.

With 15 Republicans in the race, though, it is unclear who might benefit if a candidate attacks one of his rivals more directly. And Trump’s supporters, who have embraced his unconventional tactics, may not be easily swayed by conventional advertising. Two commercials in Iowa announced last week by the conservative Club for Growth, attacking Trump as a closet liberal, were criticized by rival campaign operatives as a waste of $1 million — the wrong message at the wrong moment.

Even though voters increasingly get information from smartphones and laptops, 30-second television commercials are still king. TV spending in the 2016 elections is projected to reach $4.4 billion, up 16 percent from 2012, according to Kantar Media/CMAG, which tracks TV advertising.

And of the $106 million in advertising time reserved on behalf of Republicans in the first four nominating states, super PACs and other outside groups account for 82 percent, Kantar said.

Political ads are likely to remain generally positive for some time, strategists said. The real glut of negative ads in the 2012 Republican primary contest did not begin until December 2011 in Iowa.

"I think the race will be very much in flux in Iowa and New Hampshire on Jan. 2," said Stuart Stevens, Mitt Romney’s top strategist four years ago. "Then it’s going to be a total ‘Hunger Games.’"

Campaign operatives still marvel at how a deeply funded group supporting Romney four years ago laid waste with TV ads to a surging rival, Newt Gingrich.

Then, in the general election, Priorities USA Action, a group supporting President Barack Obama, effectively painted Romney as a ruthless businessman who did not care about the average American — an image he never shook.

Priorities, which supports Hillary Rodham Clinton for the Democratic nomination, has so far not reserved any TV time in the first four nominating states, according to Kantar Media, although Clinton’s campaign itself plans to spend over $14 million.

For the moment, with Trump monopolizing so much of the news coverage that candidates typically have relied on to introduce themselves this early in an election cycle, super PACs supporting his rivals are turning to positive ads that lay out the candidates’ records.

"A real side effect of the incessant coverage of Trump has been the elimination of that opportunity unless you are talking Trump — which is not particularly productive," said Brad Dayspring, an adviser to the super PAC set up to support Walker, which had begun a $17 million advertising spree on his behalf just days before Walker exited the race.

But TV ads alone will not necessarily keep a flailing candidate alive. The super PAC behind former Gov. Rick Perry of Texas pumped nearly $1 million into commercials in Iowa over the summer, but Perry still withdrew from the race after failing to coax enough donors to keep his campaign afloat.

Bush, polling behind all three nonpoliticians in the race and overshadowed in the Republican debates, probably has the most at stake in the gathering air war. His super PAC, Right to Rise USA, has apportioned $6.6 million in Iowa, $22 million in New Hampshire and $8.3 million in South Carolina — amounts that tell the tale of his early-state strategy.

An ad that began running on Wednesday in New Hampshire reviews Bush’s record of cutting taxes and vetoing spending as governor of Florida, and shows him declaring emphatically that he is a "committed conservative." (It also drops the often mocked exclamation point from the "Jeb" logo in the closing frame.)

But just as Bush has begun to jab at Trump on the campaign trail, his allies have said they would not shy away from more aggressive tactics.

"We certainly wouldn’t rule out making sure that we’re defending Gov. Bush’s record against any attacks from other candidates, and making sure we’re contrasting Gov. Bush’s record with those of the other candidates in the race," said Paul Lindsay, a spokesman for Right to Rise USA.

Right to Rise’s ad-maker, Larry McCarthy, produced the "Willie Horton" ad for George Bush that devastated the presidential campaign of Michael Dukakis, the Democratic nominee, in 1988. More recently he worked for Romney’s 2012 super PAC.

At some point, the super PAC may sorely strain Bush’s oft-expressed desire to campaign "joyfully."