With some Senate races starting to tighten up, the Senate majority leader, Charles Schumer, is putting some of his own campaign warchest on the line in an effort to retain a Democratic majority.

Mr. Schumer is doling out $15 million of his $38 million campaign fund to counterbalance recent Republican gains and help maintain his position as majority leader.

The money will be spread across nine $1 million donations, in Arizona, Georgia, New Hampshire, Nevada, Wisconsin, Pennsylvania, Ohio, North Carolina and Florida, and two $500,000 donations, in Colorado and Washington.

Mr. Schumer’s campaign is also giving $5 million to the Democratic Senatorial Campaign Committee, which helps support Democratic candidates across the country and has distributed nearly $130 million of the $173 million it raised this year.

Mr. Schumer’s play contrasts recent moves by the Republican side, where candidates have struggled to finance their own campaigns and party leaders have been forced to make tough decisions about where to dedicate money.

At the top of that list is Georgia, where the Republican Senate Leadership Fund dedicated $37 million. The group, which is aligned with the Senate minority leader, Mitch McConnell, also made major investments in Ohio and North Carolina, where it put $28 million and $27.5 million, respectively.

RealClearPolitics’s average of Senate polls identified both Georgia and North Carolina as races that have tightened up in recent weeks and where Senate candidates are running within a point of each other.

The polling averages also give Republicans a slight edge in Ohio, though recent polling by the Cincinnati Enquirer/Suffolk suggests the Democratic nominee, Congressman Tim Ryan, may have taken the lead.

The Senate Leadership Fund president, Steven Law, describes the race in Ohio as an “unexpected expense” that has burdened the party’s bottom line more than any other race this cycle.

A tech mogul, Peter Thiel, propped up his one-time apprentice, J.D. Vance, in the primary there, alongside another venture capitalist in Arizona, Blake Masters.

Mr. Thiel’s money, however, has dried up since the primary, forcing the party to step in. In announcing some $8 million in ad cuts in Arizona, the party appears to have decided that Mr. Vance needs the money more than Mr. Masters.

Doubling down on the party’s investment in Ohio, President Trump, who endorsed Mr. Vance in the primary, is set to appear at a rally in Mr. Ryan’s district on September 17.

Although Mr. Vance is expected to pull through in November, Mr. Ryan has been leading in the polls. Mr. Trump’s appearance could sway the race, as he won the state by about eight points in 2016 and 2020.

Mr. Schumer, for his part, must be feeling rather confident in his own race, and also in Democratic prospects generally, in a midterm election that only a few months ago had been expected to deliver majorities in the House and the Senate to the Republicans.

Now, the Republican prospects don’t look so clear, with most forecasters expecting the battle for the Senate to be close, or for Democrats to even gain a seat.

A political scientist at Hamilton College, Philip Klinkner, says that “reading the tea leaves” of public polls and money shuffling can be a fruitless task, but that it’s safe to conclude that Republicans are in a less secure position than they were hoping for in 2022.

“When Republicans thought this was going to be an easy year they were going to sink all their money into challengers, now they’re spending it on defense,” Mr. Klinkner said.

With Republican money flowing into Ohio, a seat currently held by Republicans, Mr. Klinkner said the GOP is reacting to a mixed national environment.

Democrats, having expected a “red-wave” this year, seem to be feeling good about this position, as reflected by Mr. Schumer’s statement accompanying the announcement: “Keeping and growing the Democratic majority in the Senate is my top priority,” he said.