Sen. Marco Rubio (R-Fla.) is greeted as he visits an early voting center to cast his general election ballot on Oct. 31, 2016 in Miami, Florida. Mr. Rubio is in a race against his Democratic opponent Rep. Patrick Murphy (D-Fla.) for the Florida Senate seat. (Photo by Joe Raedle/Getty Images)

With a week to go until Election Day, the outlook for the Senate remains uncomfortably cloudy.

About half of the 11 races that have been watched at some point over the last year are still too-close-to-call. But Republicans remain on the defensive, and Democrats have many routes to gaining the four seats they need to take control of the Senate.

The presidential contest remains an important cloud hanging over the fight for the Senate.

The re-emergence of Hillary Clinton’s email issue puts the former secretary of state on the defensive and gives ammunition to GOP House and Senate candidates. Clinton still has a clear advantage in the presidential contest, but FBI Director James Comey’s recent letters (one to Congress and the other to FBI employees) add some near-term uncertainty to the overall political environment.

The Arizona and Ohio Senate races, which once looked competitive, no longer do. GOP incumbents John McCain and Rob Portman have built up solid leads and will win re-election. Two Republican incumbents, Mark Kirk of Illinois and Ron Johnson of Wisconsin, have not successfully overcome early deficits and appear headed for defeat.  Johnson has closed his gap against challenger Russ Feingold, a former senator, but the Republican remains a clear underdog for a second term.

In Florida, GOP Sen. Marco Rubio leads challenger Patrick Murphy in the mid-single digits.  The race isn’t over yet, but the Republican has a clear advantage even though GOP early voting in the state has been disappointing.

Nevada, the only Democratic seat in play, remains competitive, but Republicans are very worried about the Democrats’ money advantage and the Latino vote. Historically, Republicans don’t close well in the Silver State, and early voting looks good for Democrats. Given that, and in spite of GOP Senate nominee Joe Heck’s early lead in the race, Nevada now leans to Democrat Catherine Cortez Masto.

The remaining five competitive races are being defended by the GOP.

Two of the five Republicans have run very strong races – Pat Toomey in Pennsylvania and Todd Young in Indiana.

Toomey and Democrat Katie McGinty are running about even statewide, but Toomey is being hurt by Donald Trump’s weakness in Philadelphia and its suburbs, which often produce a margin large enough to offset GOP gains in the central part of the state.

Young started well behind former Sen. Evan Bayh, whose initial favorable ratings and financial muscle made him the overwhelming early favorite. But the race has closed as voters learn information about Bayh, and Trump should win Indiana, which could help Young. Some Republicans worry that Young’s momentum has stalled, but most believe the trajectory points to a Young advantage. In any case, this race is a toss-up.

Two Republicans in tight contests continue to get terrible reviews for the races they have run: Roy Blunt (Mo.) and Richard Burr (N.C.). Democratic challengers in both races have been impressive, with Missouri Democrat Jason Kander drawing raves.

Trump will win Missouri, but Democrats have successfully branded Blunt as a Washington insider close to lobbyists. That’s not an ideal image this cycle. More than a few Republicans believe that Blunt is now more likely than not headed for defeat, though Trump’s margin of victory and the last-minute Comey controversy could impact the Senate race enough to rescue Blunt from defeat.

In the Tar Heel state, polls continue to show the race tight, with the overall political environment in North Carolina – Trump appears headed for defeat, as does the GOP governor – playing right into Democrats’ hands.

Few Republicans expected Deborah Ross to be as strong a candidate as she has been. But she still has one glaring area of vulnerability: her years heading the state American Civil Liberties Union.  Some of her positions on controversial issues (including doubts about the creation of a sex-offender registry) have helped Burr improve his position lately. But while the incumbent probably holds a razor-thin lead, this race is far too close to call.

Finally, in New Hampshire, incumbent Kelly Ayotte’s inability to explain her support/lack of support for Donald Trump could cost her another term.

Ayotte and Democratic challenger Maggie Hassan have been locked in a tight contest, which is too-close-to-call. Republicans have believed that Ayotte will run ahead of Trump in the Granite State, but they’ve worried that Trump could lose the state by a large enough margin that Ayotte cannot overcome the drag from the top of the ticket. But now, they hope that Clinton’s victory margin in the state could shrink to the low single digits, giving Ayotte a real chance to survive.

The Republicans’ problem is that, unless they win the Nevada Senate race, they’ll need to win at least four of the five tightest contests. That is possible, of course, but it is also very challenging, especially since Clinton is expected to carry three of those five states.

Comey’s letter to Capitol Hill gives GOP strategists reason for hope, and a race-by-race assessment of the competitive contests suggests that anything from a Democratic gain of as few as three to as many as eight seats is possible.

But given the much greater Republican vulnerability, Democrat gains of four to seven seats now looks most likely. And that would flip the Senate.

Stuart Rothenberg writes about the politics of the presidential and congressional races.

Read more Political Stu.