WASHINGTON (TNS) — Missouri is the top target for outside-group spenders in Senate races across the country, according to data analyzed by the nonpartisan Center for Responsive Politics.

Data compiled by the group from Federal Election Commission reports indicate that about $23 million has been spent so far, slightly over half of it by groups aligned with Democrats.

But that figure, which rises by the day, does not encompass all "dark money" spending or spending not yet reported to the FEC. For example, the Koch Brothers' Americans for Prosperity announced a $2.1 million ad buy attacking Democratic incumbent Sen. Claire McCaskill. In February, it bought $1.8 million in ads attacking McCaskill for voting against Republican tax cuts.

McCaskill's campaign, citing media reports on top of the CRP-analyzed data, estimates that at least $37 million has already been spent in Missouri outside of the campaigns of McCaskill, Republican Attorney General Josh Hawley and independent candidate Craig O'Dear. That spending, which includes the harder-to-trace dark money, outpaces the combined amount of money raised by those three candidates for their own campaigns, and is on a pace to set records for Missouri Senate races.

Of the $23 million reported to the FEC and analyzed by the CRP, $13.5 million has been spent to help McCaskill, roughly $9.5 million to aid Hawley.

The flood of outside money guarantees that Missouri's airways will be saturated with Senate-race advertising over the next eight-plus weeks. St. Louis Public Radio, citing reports to the FCC, reported last week that St. Louis stations alone are running at least $500,000 worth of Senate ads each week.

About 44 cents of every dollar of the $23 million analyzed by the CRP has been spent on advertising to boost McCaskill by the Senate Majority PAC. It is affiliated with McCaskill's colleague, Democratic Senate Leader Charles E. Schumer. That super PAC has spent more than $10.1 million so far, according to the CRP data, and it plans to spend at least a $9 million more in Missouri over the last two months of the campaign.

The Schumer PAC, like many, also has a nonprofit dark money affiliate, Majority Forward, which is also expected to spend in the state. By law, McCaskill and Schumer cannot discuss the outside spending.

That major expenditure has come as Hawley attempts to tie his opponent to Schumer, a liberal New Yorker who is a close friend of McCaskill's.

"It's not surprising that Chuck Schumer is spending tens of millions of dollars to keep Claire McCaskill in the Senate," Hawley said. "She votes with him nearly 90 percent of the time. When it comes to building a wall, supporting conservative judges and cutting taxes, she is with Chuck Schumer and against Missouri."

According to the CRP analysis, the National Republican Senatorial Committee has been one of the biggest spenders for Hawley so far, spending almost $4 million to praise him or attack McCaskill. That committee taps top Republican donors as well as rank-and-file givers to help Republicans get elected to the Senate.

McCaskill has long tried to portray herself as an independent, willing to buck her own leadership, including Schumer, when necessary.

"Certainly he has moments when he is not (bipartisan) and I tell him so," McCaskill said this summer. "In fact I have told him to be quiet and quit getting his mug all over television, that he is not always helpful."

"I support public disclosure of these donors and campaign finance reform, my opponent does not," she added. "I have been clear with Missourians that if they can't figure out who paid for an ad, then they should ignore it — whether that ad is for me or against me."

Chris Hayden, a spokesman for the Schumer-related PAC, said claims that the heavy spending had made McCaskill beholden to Schumer were belied by McCaskill's actions.

"Claire has shown through her record she is an independent senator through her votes," Hayden said. "She stands up to anyone."

The amount of outside spending in Missouri is reflective of the pivotal role Missouri plays in the control of Congress in 2019. Democrats have a chance to take the U.S. House, but their road to control of the Senate is rockier. The seat now held by McCaskill is an important part of any Senate-control equation.

"Missouri is attracting a lot of outside spending because it has been seen by leaders of both parties as a seat where a Democratic incumbent could be defeated, and that a defeat of McCaskill could help the Republicans hold a slim majority in the U.S. Senate," said Dave Robertson, head of the Department of Political Science at the University of Missouri-St. Louis. "That matters a lot because only the Senate confirms presidential nominees for positions in the Cabinet and on federal courts."

Outside spending is even more crucial for Republicans because McCaskill has raised more than four times what Hawley has raised. The latest FEC reports, as of July 18, showed that McCaskill had raised roughly $22.8 million and spent $16.2 million. Hawley had raised $5.3 million and spent just under $3 million. O'Dear raised about $454,000 and spent $314,000.