The leading Senate Democratic super PAC and an affiliated nonprofit raised $147 million from April through June, the groups said, notching its best-ever second quarter haul as the party seeks to win back control of the upper chamber in this fall’s midterm elections.

Senate Majority PAC, the super PAC that is aligned with Senator Chuck Schumer of New York, the minority leader, brought in $58 million and entered July with $126 million in its account, more cash to spend than it has had at this point in previous election cycles. Along with Majority Forward, its associated nonprofit, Senate Majority PAC is expected to spend big in this year’s marquee Senate races.

But heading into the second quarter, Democrats were by some measures playing catch-up against their Republican counterparts, and it was not immediately clear whether they had made up ground. Through the end of March, the Senate Leadership Fund super PAC, a leading G.O.P. group, had more than twice as much cash on hand as the Senate Democratic group, with an advantage of $92 million. The Republican organization has not yet announced its haul for the second quarter; the latest quarterly reports are due to the Federal Election Commission by July 15.

Democrats need to gain at least four seats to win back the Senate, an uphill climb, but one that has looked more realistic than it did at the start of the cycle. President Trump’s unpopularity and concerns about the economy have opened the door for the party out of power.

“It’s a big map, not just because of the number of states, but the size of the states,” J.B. Poersch, the Senate Majority PAC president, said in an interview. “Places like Georgia, North Carolina, Ohio — they’re not cheap dates.”

Democrats are also navigating their own internal challenges, most notably the implosion of their Senate nominee’s campaign in Maine, a key battleground. The candidate, Graham Platner, said on Wednesday that he was suspending his campaign. The move, coming under pressure from his party following a rape allegation that he denied, has left Democrats scrambling to field an alternative against Senator Susan Collins, a Republican.

Mr. Poersch said his group would spend in support of the Democratic nominee for Senate in Maine. It is not yet clear who that will be, with many candidates lining up to replace Mr. Platner.

Groups backing Ms. Collins have been running millions of dollars’ worth of television ads across Maine, and the race will only get more expensive as November approaches.

Overall, G.O.P. committees had out-raised Democrats’ main political committees by a staggering $600 million entering April, according to a New York Times analysis. President Trump’s super PAC and the Republican National Committee have reported particularly strong numbers this cycle in comparison to Democrats.

Beyond Maine, Democrats have a chance to flip seats in North Carolina, Ohio and Alaska, along with Iowa and Texas. But they are also defending an open seat in Michigan, a state Republicans have eyed as a pickup opportunity, and where a contentious Democratic primary is playing out.

Both major parties rely on super PACs, which can raise and spend unlimited sums of money, and nonprofits, which do not have to disclose their donors. The lines between the two have become increasingly blurred in both parties as nonprofits pay for waves of advertising in key Senate races.

Shane Goldmacher contributed reporting.