Senate Majority PAC, far and away the biggest-spending super political action committee of the election cycle — at nearly $25 million — has begun pouring millions of dollars into Michigan, Iowa and Colorado, signaling just how quickly Democrats have shifted to defense heading into November’s elections.

The super PAC, which has deep ties to Senate Majority Leader Harry Reid, is increasingly limiting its offensive efforts and is instead trying to create a firewall in Colorado, Michigan and even New Hampshire, hoping to halt a potential GOP wave that would net Republicans the six seats needed to capture the chamber and remove Mr. Reid from his leader’s post.

Since July 1 alone, the super PAC has spent more than $671,000 attacking Terri Lynn Land in Michigan, $824,000 against Rep. Cory Gardner in Colorado and about $1 million against Republican Joni Ernst in Iowa.

They’re even entering the fray in New Hampshire, where Republican Scott Brown is trailing incumbent Democratic Sen. Jeanne Shaheen by double digits in some polls. The group has spent about $350,000 since July 23 and $1 million this cycle on independent expenditures devoted to attacking Mr. Brown.

The investments in new territory reflect just how much the political environment has shifted since earlier this year, when Democrats had hoped to challenge two GOP-held seats in Kentucky and Georgia and to easily hold seats being left vacant by Democratic retirements in Iowa and Michigan.

“I think if Senate Majority PAC were to continue to spend in New Hampshire, that could be a firewall situation, but I think the majority would have been lost by then if we’re talking about New Hampshire going for Scott Brown,” said Nathan L. Gonzales, deputy editor of the nonpartisan Rothenberg Political Report.

Though super PACs are not allowed to coordinate with candidates or campaigns, Senate Majority PAC was founded by a former chief of staff to Mr. Reid, and its treasurer is a longtime strategist for the Nevada Democrat.

Many of its strategic decisions, most notably meddling in Republican primaries, have the imprimatur of Mr. Reid, and as the leading outside group for Democrats this year, its strategic decisions come with an air of authority.

One proof is that the Democratic Senatorial Campaign Committee, the official campaign arm of Senate Democrats, and other groups supporting Democrats have outspent their GOP counterpart and other groups backing Republicans by a combined $8.2 million in Michigan and Colorado alone, the Republicans said.

Jim Manley, a former senior aide to Mr. Reid, said Senate Majority PAC’s activity is trying to shape the field now as well as prepare for an expected GOP onslaught later.

“The PAC is doing a couple of things,” Mr. Manley said. “First of all, [they’re] trying to get involved in some primaries and stir the pot a little bit. Second, they’re trying to protect some folks [who] will probably come under air assault in the fall.”

Some of that pot-stirring came in North Carolina earlier this year, where the PAC has spent about $5.7 million on independent expenditures against Republican Thom Tillis, much of it in an attempt to boost the tea party-backed Greg Brannon in the Republican primary against Mr. Tillis.

Democrats had hoped for a repeat of races in 2010 and 2012, when insurgent tea partyers won primaries but lost the November elections, keeping seats in Democrats’ hands. But Mr. Tillis won that primary and is now mounting a spirited challenge against incumbent Democratic Sen. Kay R. Hagan.

Elsewhere, the PAC is spending $2.1 million in Louisiana to help Sen. Mary L. Landrieu and $1.9 million in Arkansas defending Sen. Mark L. Pryor. It has also contributed millions to Put Alaska First, a super PAC boosting incumbent Sen. Mark Begich, and has spent about $2.5 million in an effort to oust Senate Minority Leader Mitch McConnell in Kentucky.

Other investments are a bit more puzzling. The group recently spent about $200,000 attacking GOP Rep. Shelley Moore Capito in a Senate race in West Virginia that appears to be well within Republican hands.

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