2016 by the numbers: Where does the battle for the Senate stand?

The Capitol dome on Capitol Hill in Washington.

J. Scott Applewhite/AP

At the start of the 2016 election cycle, Republicans hoping to retain control of the Senate already had their work cut out for them. The GOP is defending 24 seats, while the Democrats are only defending 10, and most of the competitive races are for Republican-held seats – a consequence of GOP gains six years ago in the 2010 midterms.

Then, in a twist few expected, Donald Trump won the GOP nomination. With that, most GOP strategists will acknowledge, the task of keeping the Senate became even more difficult, as Republican candidates have been forced to dance around the chaos and controversy at the top of the ticket.

Democrats currently control 46 seats -  if you include the two independents who caucus with their party - while Republicans control 54. There are two possible routes to a Democratic take-over of the Senate: either Democrats must win four seats and win the White House (to allow a Democratic vice president to break a 50-50 tie in the upper chamber), or they can lose the White House and win at least five seats in the Senate.

With fewer than three months left before the Election Day, Democrats are within striking distance of their goal, but a handful of contests remain competitive enough to swing in either direction by November, so it’s likely neither party is counting their chickens.

Here’s a look at where the battle for control of the Senate stands:

Likely Democratic gains

Democratic candidates are considered the favorites in two Senate races against GOP incumbents: in Illinois and in Wisconsin.

In Illinois, Democratic Rep. Tammy Duckworth is challenging GOP Sen. Mark Kirk. Public polling of the race is scarce, but most analysts acknowledge the state’s deep Democratic tilt makes Kirk’s job very difficult. A NBC/Wall Street Journal/Marist poll released in March found Hillary Clinton ahead of Donald Trump by 25 points, 57 to 32 percent. Kirk has tried to distance himself from his party, striking an independent note on Supreme Court nominations and declaring his opposition to Trump, for example. But to win, he would have to count on huge contingent of Clinton voters willing to split the ticket and vote for a Republican senator – a tall order in an era of rigid partisanship.

An internal poll released by Kirk’s campaign in April found him trailing Duckworth by three points, 39.6 percent to 42.7 percent. But it’s saying something that the most optimistic picture of the race Kirk’s team can muster is a poll showing their candidate trailing his challenger.

In Wisconsin, GOP Sen. Ron Johnson has fallen behind former Democratic Sen. Russ Feingold, who was ousted by Johnson in 2010 and is now waging a rematch. The latest Marquette University poll​, considered the gold standard of election polling in the State, found both Feingold and Clinton ahead of their GOP challengers by double digits: Feingold ahead of Johnson, 53 to 42 percent, and Clinton ahead of Trump 46 to 36 percent.

Toss-ups

Most of the competitive Senate races this cycle remain a jump ball, ready to be claimed by either party. And in most of these close races, polls show GOP Senate candidates in a better position than Trump – a sign that individual Republicans’ efforts to distance themselves from the top of the ticket may be working.

In Florida, GOP Sen. Marco Rubio enjoys a consistent but not overwhelming advantage over Rep. Patrick Murphy, his likely Democratic opponent. (Murphy has yet to win his primary, but most analysts expect him to prevail.) And Rubio is one of those Senate candidates who’s outperforming Trump in his state, which may help him keep his seat even if Clinton wins the Florida’s Electoral College votes.

A Monmouth poll​ this week, for example, found Rubio ahead of Murphy by five points, 48 to 43 percent, but it found Clinton ahead of Trump by nine points, 48 to 39 percent. Similarly, an NBC/Wall Street Journal/Marist poll​ last week found Rubio up by five and Clinton up by six. And a Quinnipiac poll​ last week found Clinton up by one and Rubio up by three.

In New Hampshire, GOP Sen. Kelly Ayotte faces a stiff challenge from Democratic Gov. Maggie Hassan. A CBS News Battleground Tracker poll​ released Sunday showed Hassan up by one point, 42 to 41 percent. But the same poll showed Clinton ahead of Trump by 9 points, 45 to 36 percent. And a WBUR/MassINC poll​ earlier this month found Hassan up by 10 points, 50 to 40 percent, and Clinton up by 15 points, 47 to 32 percent. Again, Ayotte is in a tough spot, but she seems to be doing better than Trump.

Ditto for Pennsylvania GOP Sen. Pat Toomey, who trails Democratic challenger Katie McGinty in most recent statewide polls but is still outperforming Trump. Last week’s Quinnipiac poll found McGinty up by just three points, 47 to 44 percent, but it showed Clinton with a healthier lead, 52 to 42 percent. Last week’s NBC/WSJ/Marist poll​ found a similar spread: McGinty up by four, Clinton up by nine.

In North Carolina, GOP Sen. Richard Burr is facing a surprisingly robust challenge from former Democratic State Rep. Deborah Ross. A NBC/WSJ/Marist survey last week found Burr trailing Ross, 44 to 46 percent. But, again, he’s doing better than Trump, who was behind Clinton in the same survey, 39 to 48 percent.

In Indiana, two developments have provided Democrats with another pickup opportunity: the retirement of GOP Sen. Dan Coats, and the candidacy of former Democratic Sen. Evan Bayh. A Monmouth poll​ released Wednesday found Bayh ahead of his GOP opponent, Rep. Todd Young, 48 to 41 percent. 

Young is actually that rare Republican Senate candidate who is performing worse than Trump in his state - the Monmouth survey showed Trump beating Clinton, 47 to 36 percent. Two possible explanations: Bayh, a former senator with a moderate reputation, may be an especially strong Democratic candidate, and the presence of Indiana Gov. Mike Pence on the GOP ticket may be strengthening Trump’s hand in the state.

In Arizona, once a Republican bastion and now an unexpected battleground, Sen. John McCain is in one of his toughest fights in decades against Democratic Rep. Ann Kirkpatrick. Polls show both the senate and presidential races are too close to call, with a slight advantage for the Republicans. A June poll from Public Policy Polling​, a Democratic pollster, found McCain barely ahead of Kirkpatrick, 42 to 40 percent, and Trump ahead of Clinton, 44 to 40 percent. (A CBS News Battleground Tracker​ released last week found Trump narrowly ahead of Clinton, 44 to 42 percent, but it did not ask voters about the Senate race because McCain and Kirkpatrick had not yet cleared their primaries.)

Silver linings for GOP?

Despite the prevailing sense that Democrats are on the advance, there are some heartening developments for Republicans in a few spots on the map.

In Ohio, GOP Sen. Rob Portman drew a top-tier challenger in former Democratic Gov. Ted Strickland, but thus far Portman seems to be holding Strickland at bay, even as Clinton seems to be padding her own lead. A Quinnipiac poll last week found Portman ahead by nine points, 49 to 40 percent, while Clinton actually led Trump in the same poll, 49 to 45 percent. And an NBC/WSJ/Marist poll last week found both Portman and Clinton up by five points.

And in Nevada, the impending retirement of Senate Democratic Leader Harry Reid has provided Republicans with their lone pickup opportunity on the 2016 Senate map. The Republican candidate to fill Reid’s seat, Rep. Joe Heck, is in a dead heat with the Democratic candidate, Catherine Cortez Masto. And if Republicans prevail in the Silver State, the Democratic route to the majority becomes far more tenuous: they would need to win at least five seats, plus the presidency, or six seats if they lose the presidency.

A CBS News Battleground Tracker poll released Sunday found a very close race in Nevada. Clinton was ahead of Trump, 43 to 41 percent, but Cortez-Masto actually narrowly trailed Heck, 38 to 35 percent. 23 percent of voters were unsure of their preference in the Senate race.