The Republican field for the congressional seat now held by US Representative Niki Tsongas got a little smaller Tuesday, with one contender dropping out and throwing his support behind another.

Calling it “the best opportunity’’ the GOP has had in years to take the seat, Scott Gunderson of Groton said he is now backing Pepperell’s Rick Green.

“Rick Green has shown that he is in a position to use his leadership skills and business experience to give us the best chance to win the seat and deliver a government that works for the people of the Third District,’’ Gunderson said in a statement distributed by the Green campaign. “For those reasons, I have decided to put personal aspirations aside and throw my full support behind Rick and to help him bring fiscal sanity and accountability back to the state.’’

Green is the founder of the Massachusetts Fiscal Alliance and the CEO of 1A Auto, a company he started with his brother. Tsongas, a Lowell Democrat, announced in August she will not seek reelection, setting off a scramble for an open seat from Massachusetts in Congress.

Lou Marino of Fitchburg is also vying for the Republican nomination. Across the aisle, the pool of candidates has now stretched to 13. Keith St. John of Marlborough is the latest Democrat to announce a bid. The others: Alexandra Chandler, Abhijit Das, Rufus Gifford, Steve Kerrigan, Dan Koh, state Senator Barbara L’Italien, Patrick Littlefield, state Representative Juana Matias, Bopha Malone, Nadeem Mazen, Terry Ryan, and Lori Trahan.