ALBANY -- Republicans and Democrats are in a pitched and expensive battle for control of the state Senate, and Election Day will help determine who will win the majority in the closely divided house.

Key races are in the Hudson Valley, Long Island and western New York, and the state's fiscal policies, social issues and immigration positions are at stake depending on who runs the Senate next year.

Republicans hold a one-seat majority only because a Democrat, Simcha Felder of Brooklyn, sits with them.

But there's another twist: There's a Independent Democratic Conference that has previously aided Republicans in the majority.

Now with Senate control on the line, the IDC may play kingmaker. And the conference is growing: It had five members earlier this year, but now it's up to seven members in the 63-seat conference.

On Monday, the IDC added its seventh member, Sen. Jesse Hamilton, a Democrat from Brooklyn.

"Six years ago, our conference grew out of a mission to make positive change in this state, and today its continued growth is a validation of its success," the conference, headed by Sen. Jeff Klein, D-Bronx, who represents parts of lower Westchester County, said in a statement Monday night.

There are several key races on Long Island, as well as in the Poughkeepsie area, where Sen. Sue Serino, R-Hyde Park, Dutchess County, is in a rematch against Democrat Terry Gipson.

Republicans have held the Senate majority for most of the last half century, but Democrats went into this year's elections hopeful they could regain a majority they lost in 2010 after two years marred by scandal.

The presidential race was expected to help Democrats because of high turnout and the state's two-to-one Democratic enrollment edge.

Republicans sought to tout their record, which in working with Democratic Gov. Andrew Cuomo and the Democratic-led Assembly included income-tax cuts, a property-tax cap and record aid for schools.

They've also warned that a Democratic-controlled Senate would mean both the legislative and executive branch would be controlled by one party, often citing the pending influence of liberal New York City Mayor Bill de Blasio in attack ads.

Cuomo, meanwhile, has gotten more involved in Senate races than he had during his six years in office.

He actively campaigned for Democrats and transferred some campaign cash to key races, saying it was time for the party to control the Senate after touting his working relationship with Republicans since he took office in 2011.