A leading environmental group is targeting former congressman Joe Baca (D-Calif.) with a pair of new mailers a week ahead of an increasingly unpredictable all-party U.S. House primary that threatens to dash Democratic hopes for the second straight election cycle.

The League of Conservation Voters, in conjunction with the affiliated California LCV, released a pair of mailers casting Baca as unfriendly to the environment.

"Congressman Joe Baca Has Repeatedly Voted to Put Our Drinking Water — and Our Families’ Health — at Risk," reads one.

The California LCV has endorsed one of Baca's opponents, Redlands Mayor Pete Aguilar (D), ahead of next Tuesday's all-party primary in California's 31st district. The top two vote-getters -- regardless of party affiliation -- will advance to the general election.

The move against Baca is the latest sign he is gaining steam. A Democratic Congressional Campaign Committee poll reported by Roll Call earlier showed Baca (13 percent) increasing his support to equal Democrat Eloise Gomez Reyes's and run just behind Redlands Mayor Pete Aguilar (15 percent). The DCCC backs Aguilar.

The women's group Emily's List backs Reyes. A secretive group has reportedly been spending money for Baca. The result has been a messy primary that has pitted Democratic groups against one another and serves as a reminder that even in a district that leans hard to the left -- President Obama won 57 percent of the vote there in 2012 -- Republicans could win the seat because of the primary format.

The DCCC poll showed businessman Paul Chabot (R) leading the primary field with 23 percent of the vote. A pair of other Republicans are polling in the 6-7 percent range. So, it's not out of the realm of possibility that two Republicans make the runoff.

That's exactly what happened in 2012, when Rep. Gary Miller (R) and another Republican made it the the general election because the Democratic vote was fractured. Miller is retiring at the end of the year.

The primary is June 3.