Why Republicans are favored to keep the Senate — and what Democrats need to do to pull off an upset

Republicans have a great chance of keeping the Senate in their control with only a few days to go until Tuesday's midterm elections.

Still, Democrats have a path — though difficult and unlikely — to a majority in the chamber.

The GOP is heavily favored to hold or boost its 51-49 advantage in the Senate. Polling suggests Republicans have an edge to flip at least one Democratic seat, while the GOP either has a lead or is locked in tight races for all of the seats it will defend on Tuesday.

But a few individual contests swinging Democrats' way could drastically alter the battle for the Senate.

The chamber's partisan breakdown could have impacts that last for decades. It will not only affect health care and tax policy, but also President Donald Trump's push to stack federal courts with conservative judges for years to come.

Here's how the Senate state of play looks four days before the midterms:

• Fewer than 10 Senate seats appear truly in play, based on polling and forecasters' ratings. Those are Democratic-held Indiana, Florida, Missouri, Montana and North Dakota, and GOP-held Arizona, Nevada, Tennessee and Texas. Though they appear to be less competitive than others, a surprise in Republican-held Mississippi or Democratic-held New Jersey could also swing the majority.
• A race-by-race breakdown shows just how difficult it will be for Democrats to gain the two net seats needed to take Senate control. Even if Democrats win their two best pickup opportunities — Arizona and Nevada — they have to win at least one more challenging race in red territory. That would mean Democratic Sen. Heidi Heitkamp would have to defend her seat in red North Dakota — where she has recently trailed — or Democrats would have to win one of the Republican-leaning contests in Texas, Tennessee or Mississippi. Even in that scenario, Democrats would have to hold every other vulnerable seat such as Indiana, Florida and Missouri to flip two net seats.
• Based on current polling and ratings from nonpartisan analysts, North Dakota is easily the most likely Senate seat to change party this year. The tightest races — and the ones most likely to dramatically change the Senate's partisan breakdown if they swing to one party or the other — appear to be Arizona, Florida, Indiana, Missouri and Nevada.

• Data journalism site FiveThirtyEight's forecast gives Republicans an 85 percent chance of holding a Senate majority. It projects an average Republican gain of about one seat in the chamber. Of course, the site still gives Democrats a 15 percent chance of finding its way to the majority.

The jockeying for House and Senate seats has made this midterm election the most expensive ever. Campaigns, parties and outside groups had already spent $4.7 billion by Monday in congressional races and are projected to shell out more than $5.2 billion by the end of the election cycle, according to the Center for Responsive Politics. It tops a previous record of about $4.2 billion.

The most expensive Senate races largely reflect nonpartisan analysts' ratings of the most competitive contests. Florida's hotly contested Senate race has led the way among all federal races, with more than $175 million already spent.