Sen. Kelly Ayotte (R-N.H.) speaks to a hostess at the Puritan Backroom as she campaigns for reelection on Sept. 13, 2016, in Manchester, N.H. (AP Photo/Elise Amendola)

The Republicans' central Senate super PAC is plowing $21 million more into six key races in the coming weeks as the fight for control of the chamber moves to states more hospitable to GOP candidates.

The Senate Leadership Fund, a super PAC that is part of the American Crossroads suite of GOP big-money groups, is expanding its television buys in New Hampshire, North Carolina, Indiana, Missouri, Nevada and Pennsylvania, officials told The Washington Post. At the same time, the group is cutting back most of its planned spending in Ohio, where incumbent Republican Sen. Rob Portman has widened his lead over former governor Ted Strickland.

"The battle lines are shifting rapidly," said Steven Law, the super PAC's president and a former chief of staff to Senate Majority Leader Mitch McConnell (R-Ky.). "The Democrats' initial theory of 2016 was they were going to win the Senate majority because they were competing in states that [President] Obama won in 2012. Now their path to the majority is shifting to states that Mitt Romney won, and that’s much more of an uphill battle for them."

Law's confident assessment? "I think we will hold the majority," he said.

Altogether, the Senate Leadership Fund plans to spend $76 million in September and October -- and the total could swell larger by Election Day, thanks to mega-donors such as Miriam and Sheldon Adelson, who gave the super PAC $20 million in August. The spending blitz comes after an allied nonprofit group, One Nation, shelled out $26 million on "issue ads" touting GOP candidates between last October and August. That organization is not required to disclose its contributors.

Law said donors have flocked to protect the Senate majority because they see the fight as a "concrete, achievable goal" --  and one in which outside spending can have a clear impact, unlike in the White House race, which is "much larger, more chaotic and harder to predict."

And Republicans are now feeling much more bullish about their chances of staying in control. One major factor that has boosted GOP Senate candidates, Law said, is that voters are separating Donald Trump and his polarizing campaign from appeals made by down-ticket Republicans.

"People just see the top of the ticket being completely disembodied from the Senate races," Law said, adding: "I think most of our candidates have done an excellent job of presenting themselves as unique political figures with a distinct value proposition. They have been able to articulate who they are and what they are about, and what things they are getting done for people back home."

While Democrats are attempting to "suck these races up into the morass of the presidential race," he added, Republicans are trying to "push them down to the ground."

Here's a breakdown of where the Senate Leadership Fund is investing its funds and Law's assessment of the state of play:

Florida: GOP incumbent Marco Rubio v. Democratic Rep. Patrick Murphy

$10.8 million (no new buys).

"In our polling, Murphy has not really caught fire there yet," said Law, who said there could be additional investments in the state. "We feel the dynamics are pulling in our direction and Rubio is driving that race."

Ohio: GOP incumbent Rob Portman vs. Democratic former governor Ted Stickland

$1.2 million (a decrease of nearly $7 million).

"We love Rob Portman and we love the fact he has run a commanding race," he said.

New Hampshire: GOP incumbent Kelly Ayotte vs. Democratic Gov. Maggie Hassan

$22.8 million (an increase of $7 million), through affiliated super PAC Granite State Solutions.

Trying to get Ayotte reelected is by far the biggest investment that the Senate Leadership Fund is making this cycle. That's in part because the super PAC, through its affiliate Granite State Solutions, is one of the few outside groups backing her, Law said. On the Democratic side, Hassan has been buoyed by an array of allies, including labor unions and the Democratic Senate Majority PAC.

"The terrain is tough and Maggie Hassan is probably one of the Democrats' better recruits, but overall we think Ayotte has run a very good race," he said. "We actually feel like the dynamics in that race are starting to improve in our favor, but the one glaring consideration we had was the weight of message disparity."

North Carolina: GOP incumbent Richard Burr vs. Democratic challenger Deborah Ross

$9 million (an increase of $900,000).

"It's a very tight, competitive race," he said. "The battle at the gubernatorial level is causing some turbulence. One of Burr’s challenges is he is not that well known in North Carolina, so he is more impacted by externalities."

Indiana: GOP Rep. Todd Young vs. Democratic former senator Evan Bayh

$8.5 million (an increase of $4.5 million).

"We think Indiana is going to be the Democrats' Waterloo," Law said. "They are going to spend a fortune here, and they are going to lose."

Missouri: GOP incumbent Roy Blunt vs. Democratic challenger Jason Kander

$7.8 million (an increase of $5.3 million).

"We have been concerned about this race for a long time," said Law, noting "a general sour political environment in Missouri that raised concerns for us about anybody running as an incumbent. Kander presented an appealing fresh face."

Nevada: GOP Rep. Joe Heck vs. Democratic former attorney general Catherine Cortez Masto

$8.3 million (an increase of $2.3 million).

"One of the things we’ve discovered is Nevada voters are aware that Cortez Masto is [Sen.] Harry Reid's handpicked successor," he said. "He’s a tremendously polarizing figure. Harry Reid is both the albatross around her neck and her necessary patron. That’s an undertone for the whole race. We think it is a hard race, but we like our chances."

Pennsylvania: GOP incumbent Pat Toomey vs. Democratic challenger Katie McGinty

$7.5 million (an increase of $1 million).

"I think is some of the toughest terrain we have to fight in this cycle, but our view of the race is Toomey is doing remarkably well in the Philly media market particularly, which is the toughest part of the state, and we think we can make up significant ground," Law said.