With both Democratic candidates showing leads over the strongest Republican candidate in the special election to fill Secretary of State John Kerry’s vacant Senate seat, what happens in an April 30 Democratic Party primary could prove to be a deciding moment for the overall federal balance of power on reproductive rights.

While Kerry had voted to uphold bans on federal funding for abortion, he was otherwise a reliable vote for reproductive rights. If an anti-choice candidate sails onto the Democratic Party endorsement before the special election, the Senate will flip a seat from pro-choice to anti-choice.

Currently, pro-choice Rep. Ed Markey (D-MA) is leading anti-choice Rep. Steven Lynch (D-MA) in the polls, though trends show Lynch’s numbers lifting over time.

Rep. Markey has said he has spent three decades fighting for choice and plans to continue that record in the Senate. He has the support of the Democratic National Committee and the endorsement of NARAL Pro-Choice America. Meanwhile, Rep. Lynch draws upon a considerable base of union support.

Rep. Lynch was one of 33 Democrats to vote against health-care reform, which has provided contraception and maternity coverage and is widely considered an advance for reproductive health (though it does contain new abortion funding restrictions). He also has voted against buffer zones that would expand protections against routine harassment directed at abortion clinic doctors, patients, and staff. Though he voted against moves to de-fund Planned Parenthood, he also led an effort in the state legislature to ban abortions at 24 weeks.

Should Lynch win the primary, a base of Democratic voters that had been energized by Republican-led attacks on women’s rights during the 2012 elections could decline to participate in the June 25 general special election. Of three Republican candidates, State Rep. Daniel Winslow (R-Norfolk) says he is opposed to neither abortion rights nor gay rights. In the wake of former Senator Scott Brown’s decision not to run, he joins two other Republican candidates in having comparatively few resources for the general election in the predominantly Democratic state.

There are 10 Democratic Party primary debates scheduled. NARAL has donated the maximum allowed $5,000 to Rep. Markey’s campaign and will deploy on-the-ground organizing resources to get out the vote.