The Republican Party is giddily anticipating the coming midterm elections, believing that it has a good shot to reclaim control of the Senate for the first time since 2006. Indeed, the party has a number of reasons to feel optimistic: Democrats have to defend more seats, Republicans have a huge edge in the generic ballot, voters really aren't jazzed about President Obama anymore, and second-term midterms are historically terrible for the president's party.

Given those structural advantages, the consensus has long been that Republicans will do quite well in November; in late March, Nate Silver gave them a 60 percent chance to take back the Senate.

So much for that. Democrats now have a 59 percent chance to hold the Senate, according to the latest prognostication from The New York Times' data model. Worse for the GOP, the trend line shows Dems surging ahead in only the past few weeks.

So what happened? How did Democrats go from doom to optimism so swiftly?

There are two important factors at play here. The first is that Republicans appear to have overestimated the strength of their candidates, thus making winnable races less so. We're not quite talking a Todd "legitimate rape" Akin redux, but a similar phenomena is at work.

And for all the chatter about the political climate favoring Republicans, it may be tilting toward Democrats — especially on the issue of ObamaCare.

Let's begin with the candidates themselves. The Times has a handy rolling interactive on how the odds in individual races have changed over time. It's clear that recent, radical shifts in a couple of key contests — in Michigan and Arkansas — have given Democrats more life.

In Michigan, likely GOP nominee Terri Lynn Land has stumbled straight into the "war on women" trap. She claimed women "have a different lifestyle" and are "more interested in flexibility in a job than pay," prompting withering attacks from Democrats, the local press, and the president. She also appeared to support an abortion ban without exceptions for rape or incest, and has yet to clarify her stance. Though she responded to the mounting criticism with a commercial asking how she, a woman, could wage a war on women, her support has nonetheless eroded in recent polls.

Meanwhile, in Arkansas, GOP challenger Tom Cotton, a military vet and congressman, has turned out to be something of a dud candidate, failing to capitalize in a state where only 33 percent of voters approve of the president's job performance. As Dana Milbank at The Washington Post argues, part of Cotton's problem is that his military service "is no magic bullet," though he's focused on it to the exclusion of all else in his campaign platform. Voters "are already well aware of Cotton's Army career," he writes. "Now they want to know what he has done as a civilian."

After trailing Cotton by wide margins earlier this year, incumbent Sen. Mark Pryor now leads by double digits, according to several recent polls.

With Republicans needing to flip six seats to win the Senate, Democrats should retain control if they hold on to these two seats.

Then there's ObamaCare. Though the law is still relatively unpopular, it is becoming less so. A solid majority of Americans want ObamaCare to stay in place, and the narrative surrounding the law has changed dramatically since the administration announced this month that enrollments crossed the eight million mark.

Republicans may have shot themselves in the foot by making ObamaCare opposition the keystone of their midterm campaign strategy. By insisting for years that the law would be a freedom-killing fiasco — and then shutting down the government in a failed bid to stop it — they left themselves no wiggle room to praise or embrace any of the law's bright spots down the road.

We're already seeing this play out in the way the GOP has tip-toed away from discussing repeal as a viable option. Meanwhile, GOP candidates have been stuck with the near-impossible task of trying to explain their piecemeal — or at times hypocritical — support for ObamaCare now that their dire warnings about its fiery death spiral have been proven wrong.

Consider Land, who called for full ObamaCare repeal while backing its popular Medicaid expansion. Or look at former Sen. Scott Brown (R), who has hemmed and hawed on the issue without giving any semblance of a cogent position as he runs for Senate in New Hampshire. Though he supports Romneycare, he opposes ObamaCare because something something next question.

Then there was news this week that ultra-conservative Gov. Mike Pence of Indiana would accept the Medicaid expansion, badly undercutting the GOP's overall argument.

In perhaps an even sharper sign that ObamaCare is no longer a deadweight for Democrats, vulnerable incumbents like North Carolina Sen. Kay Hagan are wholly embracing it.

As always, Democrats' improved standing today comes with the caveat that anything can change before November. The extreme mutability of polls, as highlighted by the Times' shifting forecast, bears this out.

Yet the recent trends in individual races, as well as the national climate, suggest Democrats aren't doomed to be the minority party in both chambers next year. And if Democrats do hold on, Republicans may have no one to blame but themselves, which is perhaps another indication that the party has fundamental weaknesses that go beyond strategy.