Democrats are pushing back against a nonpartisan election forecast that predicts Republicans have a better-than-even chance of taking back the Senate this fall.

In a new memo, Democratic Senatorial Campaign Committee (DSCC) Executive Director Guy Cecil raised questions about an analysis out this weekend from statistician Nate Silver and insists Democrats are “up for the challenge” this fall.

“We don't minimize the challenges ahead. Rather, we view the latest projection as a reminder that we have a challenging map and important work still to do in order to preserve our majority,” Cecil wrote.

He pointed to Silver’s prediction in August 2012 that Republicans would take back the Senate, when Democrats actually netted three seats that November, as evidence of how easily the election-year tide can change.

And he argued that Democrats have a stronger message than Republicans heading into the midterms and noted the substantial investment the DSCC is making in turnout operations in competitive Senate seats as a sign that the party will hold its fragile six-seat majority despite a difficult political environment this cycle.

“Most Democratic candidates are out-polling, out-fundraising, and out-campaigning their Republican opponents up and down the map. We’re going to hold the majority again in November because Democrats are fighting for the middle class and Republicans are fighting for Washington special interests like the Koch Brothers, the Tea Party, and their reckless and irresponsible agenda that voters despise,” Cecil wrote.

Still, the decision by the DSCC to push back on the analysis is an indication of the difficult climate Democrats are facing heading into the fall.

Outside GOP groups, led by the Koch brothers-backed Americans for Prosperity, have poured tens of millions of dollars into attack ads focused on ObamaCare. Recent polling in those competitive Senate races has shown the tide shift toward the Republican candidates.

The party looks to have the advantage in most of the four targeted red states held by a Democratic incumbent, and Republicans have expanded the electoral map to include Colorado and New Hampshire with strong recruits. In blue-leaning open swing state Michigan, polling earlier this year showed the likely Republican nominee, Terri Lynn Land, holding a small but steady lead over expected Democratic nominee Rep. Gary Peters.

But Cecil declared in the memo that the election-year conversation has been “one-sided” because of the millions spent by GOP outside groups and that Democrats are ready to respond.

“The Koch brothers and other Republican allies have spent months outspending Democrats, presenting only one (false) side of the story. Many of our candidates and allies are now starting to advertise and the DSCC will spend considerable resources making sure that voters see a clear choice in this election,” he wrote.

And Cecil argued that Republican candidates could undermine their chances in some of those races, just as weak candidates did in a handful of races in 2012.

He concluded by touting the party’s chances in a number of open states and the two states they’re hoping to flip, but notably made no mention of South Dakota, an open Democratic-held seat where the party failed to recruit a strong candidate.

“It’s clear that Republican Senate candidates, even candidates favored by Washington insiders, are pandering to the far right and embracing the reckless and irresponsible agenda of the Koch Brothers that will prove costly in a general election. Democrats have strong incumbents, great recruits in Michigan, Iowa, West Virginia and Montana, and are playing offense in Kentucky, Georgia, and Mississippi,” Cecil wrote.