Six weeks before Election Day, Republicans who once hoped to bolster their narrow majority in the Senate are now retrenching in hopes of holding on, buffeted by a worsening national environment and big-spending Democrats exercising their financial advantages.

Democratic outside groups are preparing a final onslaught of advertising that Republicans privately concede will outpace their own efforts.

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That spending will exacerbate a spending advantage Democratic candidates have held in most states over their Republican rivals.

A review of campaign spending data to date shows Democratic candidates and outside groups have outspent Republicans and their allies in ten of the 14 most competitive races on the map this year — in some cases by millions of dollars.

Over the next six weeks, that spending gap will get worse: In the nine states where the major outside groups are playing, Democratic groups will outspend Republican organizations in all but two, by a combined margin of more than $40 million.

“Republican candidates rarely lose for lack of money, but clearly mega-donors have flipped a traditional Republican advantage into yet another headwind,” said Bruce Mehlman, a GOP lobbyist who closely tracks the electoral landscape.

The group, America First Action, will spend heavily in Missouri, North Dakota, Montana and Indiana, all states where Democratic incumbents are vulnerable, said Brian Walsh, its chief strategist.

The four largest outside groups — the Democratic Senatorial Campaign Committee, the National Republican Senatorial Committee, the Democratic-backing Senate Majority PAC and the Republican-backing Senate Leadership Fund — will spend a combined $150 million on television advertising in the last six weeks of the election season, according to several sources watching the advertising market.

Nearly two-thirds of that money — $96 million — is coming from the Democratic side.

In both parties’ cases, the outside super PACs are spending far more than the party committees, a reflection of the declining power of parties in a post-Citizens United world.

“We have a money problem in the Senate,” Walsh said. “Of course, this discrepancy will have an impact. We can be outspent, but not entirely flanked.”

Republicans are spending the most, more than $11 million each, in two states where they currently hold vulnerable seats — Nevada and Arizona — a sign that their priorities lie in protecting the majority.

“Republicans are guarding the goal line and protecting a certain number of seats,” said J.B. Poersch, who heads the Senate Majority PAC.

In three of those five states, Democrats will spend millions more on television than Republicans will. In the two states where Republicans have a spending gap, Tennessee and Arizona, the margin is less than $1 million each.

“Republicans are on the doorstep of getting rid of the most liberal red state Democrat in America,” said Gregg Keller, a Missouri-based Republican strategist who backs Attorney General Josh Hawley (R) over McCaskill.

“But not if we take our foot off the gas in the closing weeks. These paid media numbers are disturbing and our folks need to step up or miss out on a golden opportunity in Josh Hawley.”

Democratic outside groups will spend nearly $24 million on television ads in the last six weeks of the election, while Republican groups have largely left Scott on his own.

Other states where Republicans once saw promise are now fading from the map.

Both sides are eyeing late-developing states that may present unexpected opportunities.

Democrats have nearly $8 million in paid advertising queued up over the next six weeks, while Republicans plan to spend a little under $2 million. Several outside Republican groups are contemplating late buys, but those purchases have not materialized.

But Democratic groups have conducted polls there in recent days, hunting for an opportunity to get involved.

The two sides have spent a total of $600 million on the 14 most competitive Senate contests this year — the ten Trump states where Democrats are seeking reelection and the four Republican states that offer Democrats the best opportunities.

Democrats have outspent Republicans in all of those but Tennessee, Indiana, Pennsylvania and Florida.

With 40 days to go, Florida’s contest has already cost a combined $137 million, surpassing the most expensive Senate race of 2016, when the two sides spent $135 million over a Pennsylvania seat.

And Democrats and Republicans have spent more than $70 million in both Missouri and Nevada, nearly $60 million in Indiana and almost $50 million in Arizona.