How many seats do the Democrats need to win the Senate?

When voters head to the ballot box over the next six days, it's not only the White House that's up for grabs.

Control of the Senate is also in the mix, and its outcome could seriously impact the governing capability of the next president.

Republicans currently hold the majority with 54 seats, but Democrats are within striking distance of regaining control.

If Democrats win five seats, they win outright control of the chamber. If they win four seats and the White House, the Democratic vice president would be the tiebreaker, effectively giving Democrats control.

Republicans have a heavier lift to keep control. They're defending 24 of the 34 seats up for re-election this year. And many of those seats are in battleground states that President Barack Obama won at least once in his 2008 and 2012 presidential bids.

Democrats, on the other hand, are defending 10 seats. But only one of them is actually competitive: a seat in Nevada being vacated by retiring Senate Minority Leader Harry Reid.

Hillary Clinton, who remains the favorite to win the White House, has been pushing for a Democrat-controlled Senate. Without one, getting new legislation passed would likely be extremely difficult for her.

Republicans are already vowing to block any Supreme Court nominee she'd put before the Senate, whose constitutional duty is to advise and consent.

With an aging court already down a member following the death of Justice Antonin Scalia in February, that could create a constitutional crisis.

So far, models show Democrats as the favorites in winning Senate control.

That outcome, however, will be largely contingent on both Democratic turnout and final margins in many of these battleground states.

Expect to hear a lot from both parties about the importance of Senate contests over the next six days.