Concord — At least one-third of the state’s Senate seats are up for grabs this November, as eight members have announced they will not seek re-election.

Control of the upper chamber now is in play this election cycle, as open seats are more competitive than those with an incumbent running.

“When you open up seats, it changes that opportunity,” said Senate Minority Leader Jeff Woodburn, D-Dalton.

Republicans have led the 24-member chamber since 2010. While the party currently has a 14 to 10 edge over Democrats, six of the eight members who are leaving this year are Republicans.

Democrats see an opening, and they are mounting an effort to take back the majority. But Republicans are confident they will maintain control.

“We have every expectation we’re going to be competitive in each of these (open) seats,” said Senate Majority Leader Jeb Bradley, R-Wolfeboro. The filing period for all state candidates opens Wednesday and closes June 10.

Lawmakers next session will tackle a number of issues, including whether to again reauthorize the state’s Medicaid expansion program. Talks about how to attract young people and new businesses to the state also are bound to be on the Senate’s radar.

The six Republicans and two Democrats not seeking re-election are leaving for a variety of reasons. Some are running for higher office, and others are simply retiring.

Most of the Republican senators vacating their seats are moderates who have broken with their party on major issues, including transportation funding and health care.

Four of the six Republicans who joined with Democrats this year to reauthorize Medicaid expansion are leaving. The two Republicans who voted against repealing the so-called buffer zone around abortion clinics won’t be in the chamber next year.

“We’re losing key members,” said Sen. Lou D’Allesandro, D-Manchester, who is serving his ninth term and plans to run again. “What we’re seeing are the poles being elected, and the middle is being left out. That’s happening nationally, and we’re beginning to fall into that.”

Some chalk the departures up to a declining level of collegiality in the Senate and increasing polarization. A few of the moderate Republicans leaving the chamber have faced fierce primary challenges from the far right in recent elections.

“That increased partisanship and the expectation of some voters, they want the more partisan tone … it harder to be a moderate,” said Wayne Lesperance, a political science professor at New England College.

Two of the senators are leaving to seek other office. Republicans Jeanie Forrester and Russell Prescott are running for governor and Executive Council, respectively.

And Sen. Jerry Little, R-Weare, has been confirmed as the state’s new banking commissioner.

Other lawmakers are retiring, many for personal reasons, they said. Those retiring include Democrats Molly Kelly and Lebanon’s David Pierce, and Republicans Sam Cataldo, David Boutin and Nancy Stiles.

Stiles, who served three terms in the Senate, said she is looking to spend more time with her family.

The Senate has become more partisan, she said.

“When I first started, it seemed like one big group that kind of worked together,” said Stiles, of Hampton. “It hasn’t quite seemed the same in this last session.”